My oo guage custom engins

You sure are ambitious I'll give you that, I would have thought a good chassis to use would have been a Hornby 3F (often referred to as a Jinty), these are freely available, generally good runners and quite price efficient. They are also a 0-6-0 and not too complex both in design or workings, one reason they run nicely and are probably one of the most bullet proof models out there.

What type of 3D printer are you thinking of getting in the long term? FDM (like an Ender or similar) or an SLA (like a Mars or similar)? The reason I'm asking is you may well find printing something like that cow-catcher will be problematic at best on an FDM printer. They do not handle that style of part well, especially if you intend to print it as one piece. Of course you could print the main body as one part then the cow-catcher and similar detail parts as separate items you then subsequently add to the main body. Even commercially made models will do this, mould the main body as one piece and then add the detail parts separately, its one reason why the latest high detail models are so expensive - paying someone to fit all these miniature detail parts by hand is slow and delicate work.

Anyway keep at it, I'll be following your post with interest