Which year do you have? Big difference between 98 and 99 p80s.
On the turbo, make sure it will fit or you will likely have to do a custom exhaust manifold.
Also think about how you will route water/oil lines.
Rod, pistons, is a good idea depending on power goals, obviously if you are already in there doing that, and depending on what engine you decide to go with, do shims as well.
Upgrade the fuel pump and injectors, look at e85, and you should look into water/meth injection (kits can be had fairly cheap).
Obviously you will want to do tuning, no idea who tunes anymore, there is a good bit of knowledge out there about tuning on motronic now so depending on your knowledge you could do it yourself, maybe.
You've likely already thought about exhaust, but take into account stopping power as well as handling.