I've hit another character archetype roadblock in the "classless" system.

I really want to make a techmarine. There is no legit way by the rules to allow for this rudimentary and *canon* character archetype outside of rigging up your own cost and rules to it.The mechadendrite use/ sevo-harness is an important part of the tech marine.

Any one have any suggestions of a starting package for the tech-marine archetype?

I'd take the Sorcerer as a rough guideline, turn all his skills and talents into xp by looking up the Allied costs, completely replace his Psyker trait, Psy Rating and the 500xp of starting Powers in exchange for a Mechanicus Implant Trait, a Servo Arm or Servo Harness and a Mechadentrite System.

Change the equipment into something vaguely at the same level of availability and you should be fine, but don't treat him as having the Techpriest/Heretek discount for Cybernetics and Implants at character generation.

This way you are paying with some xp to get a unique character class tailored to your tastes which is Marine as well as "Techie", also not able to boost yourself with quite the same obscene amount of "pick-whatever-you-like" Implants and Cybernetics the Heretek can easily start with, therefore conserving his validity, though obviously if you're really lucky with your loot and equipment upgrade opportunities in game you'll start closing that gap.

This is only my suggestion, obviously you'll need to speak things through with your GM, but if you're willing to up-front pay for your concept with flexibility and a bunch of xp, I don't see any reason not to allow it.