Question for the turbo gurus out there

[Halon]

It's sounds like you want to keep cost to a minimum. So I'd say if everything is already setup to mate up perfectly to a GT42, then just stick with that platform for now for simplicity sake so it's just a bolt up affair without having to tweak piping and what-not. Little tweaks here and there can add up cost wise.

Without knowing what your current turbo is, or what exactly is wrong with it (you mentioned buying a center section, then mention a chipped wheel, what all is wrong with it?), it's hard to say what's the best path to go as far as repairing or buying new.