depends on the transmission you have. if you have a 6spd, you will just need a new clutch setup. if you have a auto, you'll probably have to get a level 10 tranny upgrade.
as far as the engine goes, it depends on what size turbo you go with. if you go with a turbo smaller than a t-63/sp63 you can probably get by with just the stock fuel system, but i'd recommend atleast a vpc, and fmic. if you go bigger you'll need to upgrade the entire fuel system. maybe some engine internals depending on what size you with. but i have heard of people slapping on a t78 with stock fuel, stock motor, but can only turn it up to about 14psi, i don't recommend that. personally the most important thing about going single is FUEL. and i bet most people would agree with me.
you might want to rebuild the motor though IF your car has a lot of miles. hope this helps.