I picked up a VersaSpot 7000 last winter, haven't had a chance to hook it up.
Purchased used and the seller gave me a quick lesson while still energized.
It not only pneumatically pinched and welded the overlap joint but was set to pulsate, made a great looking factory spot weld.
I paid $1600 for it used and it included an extra set of long arms.
When I was looking, I did occasionally find other reasonably priced spot welders on Craigslist.
Just make sure they're single phase, providing you don't have 3 phase in your shop.
Good luck.
http://www.800usaweld.com/products/spot_welders/versaspot.html