The 5.0 is the best option as everyone has been saying. You will be able to find all the parts and in my opinion v8 are the easiest engines to work on especially the 5.0.
Well I did it!!! I bought myself a mustang on friday! Its an 84 notch, has the 89 front end on it though. Its got a 5.0 and its 5spd. Got alot of work to do to it but I'm quite happy! Cant go wrong for $850.