Hi great find and very brave rebuild
I would use the xr2i efi engine ,
the engine block is very similar to the rs turbo block ,
buy some used rs turbo pistons ,
fit them in the xr2i block ,with new main and big end bearings
specify rs turbo bearings , (1988 onwards)
drill the block for oil return or add a return into the sump(above oil level ) use the xr2i wiring loom convert it to run ofab or ofac ecu, it will save many hours and dollars and be more reliable, info on the conversion is all over the internet


the lsd is standard on all mk4 rs turbo, the way to tell is look onto the gearbox where the driveshaft has been removed a mk4 turbo box has a 15mm hole inside, you can just reach it with your finger, ,
take a picture if you are unsure,
4 door shell is very useful for rebuilding your car, good luck