2500 would be alot for a used motor from what I've been seeing...since i've been thinking about it too. I'm going to just rebuild mine, and thats about what it will cost me with the upgrade to stage 2 heads(patriots)
as far as used theres a ton of motors on ebay some with low mileage, but figure you'll need to tack on 300 or so for shipping.
you can also try mustangs unlimited, they sell the sohc for 3200 (500 core included) so after you give them back yours its 2700. and that I think its brand new, not reman. but I could be wrong on that.