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Sorry, dad.

Anyways, I really like the idea of this "modular construction kit" - it reminds me a little of the rules for designing your own guns in Shadowrun. Gotta be careful not to have stuff result in something too powerful, but I'm sure that should not be a problem if you add limits and/or penalties. It shouldn't just be a price thing (else the "best" gun will always be one that has everything, rather than it being a situational subject) but feature various drawbacks, from reduced reliability or range to increased reload or cooldown times, for example. You've already included the basics in your idea; I'd just caution against the possibility of compensating any and all drawbacks with cash.
Another option would of course be to impose limits on "traits" that would simultaneously crank up the rarity, with not even the rarest (and most expensive) having enough "slots" for all upgrades?

Something that once came to mind whilst looking at Tau stuff was also the use of tabletop profiles to create a ruleset more closely resemble the TT impression. At first I only wanted to "fill some gaps" and use the TT stats as inspiration for missing guns and equipment, but now I figure you could as well reset the entire list, just to make sure everything fits in.

This essentially involves taking one or more "Rosetta Stones" that are present in both the existing P&P RAW (or has been made up by yourself) and then looking at how they are represented in the TT, before extrapolating what the numbers mean and how they would translate from one ruleset into another.

I believe it has an official profile in one of the supplements now, but the above was just to exemplify the basics of this idea I had. I'm sure it could be refined further. Provided you actually want to tweak the numbers at all, of course.

Yeah, in my design notes, there are actually stat drawbacks to most of the alterations and upgrades. For example, adding the capability of full-auto fire to any weapon reduces its Reliability by a step. If it was listed as "Reliable" it loses that trait, if it was not, it now gains the "Unreliable" trait. If it was already "Unreliable", it gains "Really Unreliable", meaning it jams on any d100 roll of a double (11, 22, 33, etc.) as well as 96+.

For the plasma weapons, adding the "Maximal" firing mode option increases the chance and severity of "Overheating", and I might actually create a new, worse Overheat trait called "Gets Hot!" that might just inflict 1W of Fatigue, per round, ignoring AV and TB, for 1d3 rounds after the rule is invoked as the weapon bleeds off excess heat.... into the user's face. Or maybe it will just blow up and (probably) kill the owner by dealing 1d10 E damage per shot remaining in the weapon (at the plasma weapon's regular Pen, which can be as high as 12).