The engine and transmission are most likely coming off of a MY03 WRX. Not sure exactly yet since the car has not been located (have somebody looking right now).
I don't think I have much beef with installing the turbo-back or the up-pipe after everything is in. I've done them before. A couple hours to do either of them, tops. Provided I don't get another Vishnu pipe that is too short by a couple mm.
Being that the clutch/flywheel will almost be brand new, I wouldn't have to get back into there and touch either of those in a long while. So, if I am going to do the flywheel, it would either be before the engine goes in or it would be an exclusive install later. However, dsmawd's description of the flywheel is just about spot on with mine. Always wanted to own a car with a lightweight flywheel, but is it needed? No.
It's not like I _have_ to do any of these. I was trying to get a feel for what people did and what they wish they would have done. I haven't decided if I want to go barebones install first or upgrade the things that would be easily done with the engine apart.