I have an Iron Horse from Nashbar and really like it, except the fork (no damping or anything, a little flexy, but it works). I re-lubed all the bearings and the fork with green auto grease, and it's running smooth. I put a Dax friction drive on it, and it had rack eyelets that worked out well.
If the bike you are looking at is not dinged up and the components aren't worn down, $150 should be ok. See if they will do $100 or $125. Check the chain with a 12" ruler as a starting point. Measure for your engine kit before you buy. And don't forget to get rid of the dinky rubber rim strips! (gorilla tape OWNS).