The preferred ruleset to play DARK•HERITAGE is d20 Modern + d20 Past + Urban Arcana using the Shadow Stalkers campaign model and the E6 capstone. d20 Past is not open content, so you actually have to have that book, or if you ignore it, just use the Shadow Chasers campaign model (which is the same thing) and ignore any feats, class abilities or skills that are obviously irrelevent to a technology level that's more or less equivalent to the Old West or the Golden Age of Piracy.
Other than d20 Past, the open content is available via the Modern SRD here, or via searchable html here. The specific list of how to modify it is also here on the wiki. E6 is listed here. And add the Madness rules, and the DARK•HERITAGE races, and you're mostly good to go.
How does E6 work on d20 Modern? Obviously the advanced classes will only show a few levels. Class abilities for advanced classes (or core classes, for that matter) can be converted into feat strings. Class features converted to feats must be taken in the order that they appear on the class list (although that doesn't mean that a character can't take other feats in between converted class ability feats); i.,e. in other words, the prerequisite for any given class ability converted to a feat are the other class abilities that precede it on the list of class abilities (the exception being bonus feats class abilities.)
Magic is obviously not going to be very D&D-like. In fact, it's going to be mostly Incantations. You can do a bit with the Occultist advanced class, too. To do just a little bit more with magic, I'd also make three Rogue abilities from Pathfinder into feats. Because I've got the OGL on the blog too, I'll reproduce them here.
This will reap the flavor benefits of the E6 system plus the flavor benefits of the Shadow Stalkers campaign model. Since this will naturally not play much like D&D (and in fact, many D&D exclusive players will, in many ways, believe simply that it's impossible to play this way--which is patently false, but neither here nor there) you'll need to remember that this is is, as the tagline of the blog says, a game that reuqires a Call of Cthulhu paradigm. Don't be fooled by the fact that it uses d20 rules into thinking that it's D&D, or that it plays like it either one.
Minor Magic: A character with this feat gains the ability to cast a 0-level spell from the mage spell list. This spell can be cast three times a day as a spell-like ability. The caster level for this ability is equal to the character's level. The save DC for this spell is 10 + the character's Intelligence modifier. The character must have an Intelligence of at least 10 to select this talent.
Major Magic: A character with this feat gains the ability to cast a 1st-level spell from the mage spell list two times a day as a spell-like ability. The caster level for this ability is equal to the character's level. The save DC for this spell is 11 + the character's Intelligence modifier. The character must have an Intelligence of at least 11 to select this talent. A character must have the minor magic feat before choosing this talent.
Familiar: A familiar is a normal animal that gains new powers and becomes a magical beast when summoned to service by a sorcerer, witch, wizard or whatever label you wish you use for magic-user.. It retains the appearance, Hit Dice, base attack bonus, base save bonuses, skills, and feats of the normal animal it once was, but it is treated as a magical beast instead of an animal for the purpose of any effect that depends on its type. Only a normal, unmodified animal may become a familiar. An animal companion cannot also function as a familiar. You can use any Small or smaller animal with a CR of 1 or less as a familiar. Unlike in D&D and other d20 games, gaining a familiar does not affect the stats or abilities of the master in any way; it's a good enough benefit to have one as it is.
Familiar Basics: Use the basic statistics for a creature of the familiar’s kind, but make the following changes:
Hit Dice: For the purpose of effects related to number of Hit Dice, use the master’s character level or the familiar’s normal HD total, whichever is higher.
Hit Points: The familiar has one-half the master’s total hit points (not including temporary hit points), rounded down, regardless of its actual Hit Dice.
Attacks: Use the master’s base attack bonus, as calculated from all his classes. Use the familiar’s Dexterity or Strength modifier, whichever is greater, to get the familiar’s melee attack bonus with natural weapons.
Damage equals that of a normal creature of the familiar’s kind.
Saving Throws: For each saving throw, use either the familiar’s base save bonus (Fortitude +2, Reflex +2, Will +0) or the master’s (as calculated from all his classes), whichever is better. The familiar uses its own ability modifiers to saves, and it doesn’t share any of the other bonuses that the master might have on saves.
Skills: For each skill in which either the master or the familiar has ranks, use either the normal skill ranks for an animal of that type or the master’s skill ranks, whichever are better. In either case, the familiar uses its own ability modifiers. Regardless of a familiar’s total skill modifiers, some skills may remain beyond the familiar’s ability to use.
Familiar Ability Descriptions
All familiars have special abilities (or impart abilities to their masters) depending on the master’s combined level in classes that grant familiars, as shown on the table below. The abilities given on the table are cumulative.
Natural Armor Adjustment: The familiar gets a +1 natural armor bonus when the master is 1st or 2nd level, a +2 natural armor bonus when the master is 3rd or 4th level, and a +3 natural armor bonus when the master is 5th or 6th level.
Intelligence: The familiar's intelligence is 6 for a 1st or 2nd level master, 7 for a 3rd or 4th level master, and 8 for a 5th or 6th level master. On rare occasions where the natural stats for the animal that is serving as a familiar is higher than that, use the higher number.
Alertness (Ex): While a familiar is within arm’s reach, the master gains the Alertness feat.
Improved Evasion (Ex): When subjected to an attack that normally allows a Reflex saving throw for half damage, a familiar takes no damage if it makes a successful saving throw and half damage even if the saving throw fails.
Share Spells: At the master’s option, he may have any spell (but not any spell-like ability) he casts on himself also affect his familiar. The familiar must be within 5 feet at the time of casting to receive the benefit.
If the spell or effect has a duration other than instantaneous, it stops affecting the familiar if it moves farther than 5 feet away and will not affect the familiar again even if it returns to the master before the duration expires. Additionally, the master may cast a spell with a target of "You" on his familiar (as a touch range spell) instead of on himself.
Deliver Touch Spells (Su): If the master is 3rd level or higher, a familiar can deliver touch spells for him. If the master and the familiar are in contact at the time the master casts a touch spell, he can designate his familiar as the "toucher." The familiar can then deliver the touch spell just as the master could. As usual, if the master casts another spell before the touch is delivered, the touch spell dissipates.
Speak with Master (Ex): If the master is 5th level or higher, a familiar and the master can communicate verbally as if they were using a common language. Other creatures do not understand the communication without magical help.
Improved Familiar: If a character takes the Familiar feat, but does not select a familiar right away (or if he dismisses his familiar, or his familiar is killed) the character may take the Improved Familiar feat and get a more powerful familiar. An improved familiar works exactly the same as a familiar, except that it does not need to be an animal; any small or smaller creature of CR 2 or less can serve as a familiar. The "classics" are imp (or quasit), pseudodragon, and homunculus--but any creature of CR 2 or less (including templates, if any) can serve as an improved familiar.