Mike,
I picked up my '77 with a 3.2 that had been done about 2 years before I bought it. The car has no rust and needed a paint job, windshield and a bunch of misc items to bring her back to a very nice (not concourse) targa.
I paid 10,000 US over a year ago and did alot of the work myself, depending on the asking price you could do a 3.0 or 3.2 swap for about 4,000 - 6,000 if you do most the work your self. I think even a top end rebuild for those are around $4,000 depending on shape.
If it's a clean car that doesn't need much other than engine work and you can get it cheap enough Go for it, they're alot of fun to work on as long as you don't go overboard
Rick