Agree with just about everything mmahamm said above, however I think it would be worth a few more bucks to ask your mechanic to drop the trans pan and have a look at the fluid. It should be clean and red, and the pan should have just a minimal amount of sludge on the bottom and around the magnet. If, by some chance, there is any amount of metal fragments in the fluid/pan, then I would bail on it. If the fluid looks ok, clean the pan, button back up, and replace the ATF+4. If there is little rust on the van, I think it's worth the 1,500+ for a rebuilt motor. Can't imagine anyone giving you much for it with #4 blowing smoke.