Don't buy that turbo, or any second hand turbo for that matter. If you are tight arseing on the the turbo you had better stop what you are planning now, as it will easily cost twice what you are thinking anyway, and the turbo selection is the most critical part.
RB swap. lol. You have a better base for power in your car at the moment, without modifying your car completely. Best engine to swap in would be the VQ25det (or VQ30det), or simply build your engine if you plan more than 250kw.
Cutting and welding cast manifolds will end in tears, perhaps a custom steampipe set of manifolds would be better? But your budget doesn't allow for that...