I just bought a 1982 5.0.  I would like to swap the 4 lug for a 5 lug. Is it possible? I would like to convert to disk brakes all around. Or would I be better off keeping the 4 and just doing the brakes.

What's a good suspension kit?

http://mjbobbitt.home.comcast.net/mustang/5lug.html

This will get you started.  Don't know if its accurate or not.  I'm going thru the same dilemma with mine.  Wheels and tires will be 800 to 1200 plus the conversion.  I priced min conversion at less than 200 parts $, me doing the work.  Still I haven't done it.