What about making a plate to adapt T3 manifold to the IS38 golf r turbo? Pretty sure its good for at least 30whp more than K04-064 and could be picked up reasonably used??
Why not just take the IS38 golf or seat cupra (even cheaper) in the first place? They're not that expensive anymore.
By the time you're done with your 1.8T build, DSG etc, you probably will spend close to as much as it costs...
I'm all for building fun stuff, but when the thing you want literally already exists and it costs about as much as you would spend building it (and it works better), then no point to build.
If you want to swap it into something, there's S18 immo off solution on my site. There's several Mk1 Golfs driving around with that solution and the Golf R motor, and that does 370hp bone stock with just a remap and 500hp with just a turbo upgrade, with acceptable spool. DI and adjustable cams/lift do wonders.
This applies pretty much to every engine. From a certain turbo size it's not usable anymore outside of a straight line with launch control and NLS.
It's a good idea to stop way before that happens, as after that you're making the car feel worse and objectively slower in most situations.