My thoughts would lean toward buying a complete car that you could hear run, and possibly drive.  A lot depends on your available storage.  I found a fox Mustang GT for $300 that had an untimely meeting with a deer.  I pulled the engine & T5, and the associated harnesses, and scrapped most of the rest.  At this point I wish I had taken a lot more out of it.  If I was looking for a quick swap, I would want to know it runs, and would want to do the comp test, and see the quality of the fluids, etc.  CL is your friend.....

I guess I should have mentioned that I've already replaced all the steering, front brakes, and the rear end, wheels & tires, replacing with v8 capable components, in anticipation of swapping.
Now I'm ready for the engine & tranny, I just don't want to get taken by someone that knows more about confirming that the engine I'm looking at really is a roller 5.0 and wc t5.  I've read quite a bit online, both here & other sites about identifying the parts, but it's not second nature to me, so I could easily make a mistake.  Like I don't really know how to tell if an uninstalled t5 is worn or not.  I don't have any fear of disassembly and rebuild, but I don't know exactly which point is walk away wear, vs. simply replace a few pieces.
I don't really have storage for another vehicle, although maybe it'd be worth it to rent a storage garage for a donor while I pull parts.