Well, an engine

If you get all the ancillaries with it (basically you want the turbo, exhaust & intake manifolds, AFM and all ECU's), then you'll pretty much have everything. I think they're most commonly in Citroen XM's if I remember correctly. There was one on eBay recently so I'd keep an eye on there for an engine. I seem to recall it went very cheaply (was thinking of buying it for my 305 but gone off the idea). I have to say, the turbo conversion would certainly make an interesting change from the standard 16v conversion, but I think it would be somewhat harder. First off, you have all the extra parts and stuff to fit in the engine bay. You might consider custom-building the pipes from the turbo to the air intake, with an intercooler over the top so you've got a reason to stick an air scoop on the bonnet, if you like that kind of thing

You'll really need to look at an engine and figure out how it'll all fit. You have a bit more to worry about on the exhaust manifold than just where the downpipe pipe goes.
Chris.