I kind of have to question used engines... who says that they are in any better shape than what you have now? Even low milage motors can have serious problems. I got my rebuild kit from Mach V and have been very happy thus far with it. I got new oem pistons, oil pump and front cover, belts, full gasket set, and a complete bearing set. not bad for $500! Machine work was the kicker... about $650 dropped on that... but there are definately places that are cheaper than where I had my work done. I had the block cleaned, honed, crank polished, and the head completely redone. You might want to give Josh at slopony a call and just have him give you a quote. IMHO, I'd rather spend a little more cash now, and have a motor that I KNOW will last another 100k+. Even a JDM motor can run you upwards of $1200-$1500 pretty easy, and again, no garuntee (at least long term). I would bet that you could get a motor rebuilt for around $900-1000 if you were careful with the money.
Just my .02 others may have a better idea. Six bolts in excellent shape are a rare find these days.
Hope this helps.