When buying an engine, ALWAYS hear it run, if you aren't confident what to look for, take someone who does, it is worth paying someone to be certain, do not accept a sellers word that the engine is good, if you can't hear it run, walk away. I am an experienced engineer and I accepted someones word,( a supposedly reputable company), I paid £500 for a Citroen diesel engine that had reputedly done 60,000 miles and had it fitted and serviced with a new belt, oil, filters, injectors overhauled, clutch and starter motor, by a Citroen specialist, the total bill was over £1200, it was delivered before Christmas and blew up on 27th January, having covered less than 1000 miles but outside its 30 day warranty.
When we stripped the engine, it had quite clearly been overheated at some point, the cylinder head was cracked and all 4 pre combustion chambers were cracked and it had suffered a partial siezure, as the piston rings were broken. The seller didn't want to know, it was out of warranty.
REMEMBER 95% of car smashes are front end bumps and the first thing that gets damaged is the radiator and the last thing anyone thinks to do, is switch off the engine, result, engine overheats and is scrap. I didn't obey my own golden rule and it cost me dearly, DON'T LET THE SAME THING HAPPEN TO YOU.