Threes not many options for oil getting in. You said catch can was dry, so that leaves the turbo. look at turbo inlet compared to outlet, should get pretty obvious pretty quick!
turn on engine and point gun at the pulley, they should be heaps obvious then. Also you should use a plug lead between the coil and plug to connect gun to as the loop can cause issues.
that's also a huge range! It really comes down to what engine characteristics you are chasing. for a street car more torque pretty much always makes a quicker nicer to drive car. I know I rather a 300kw rb2530 than rb25 due to the far improved driveability. ( and that's why I went for a 30 bottom end)
The clutch lining up part only matters to get the in put shaft from the gear box to go into the splined hole. If it's not aligned you can't get the box on. Just thought I would add that as you mentioned the clutch was aligned twice.
Have you put r34 brakes on an s2? I am asking as I have an s2 rs (rwd) that I have put r34 gtt brakes on. The bolts were m12, r34 brakes are m14. I machined new m12 inserts for the r34 calipers. perhaps rs vs rs4 is different?
Yeah I would definitely drive it. If you can hear it leaking at idle I would just block up the hose, no way would I spend that much money on one! wrecker or some super glue to the rescue.