You should let us know your budget. If you want to do a proper rebuilt that will last a long time. Or a quick refresh on the cheap.
The valuable parts of the M10 are cylinder head, block with front covers and crank shaft. The rest are pretty easy to replace or recondition.
I assume you want a 2 liter not a 1.8 for the most HP. The cylinder head is most important so no cracks, not too thin, no heavy corrosion in the water passages. Of course you can not really evaluate a cylinder head with out removing it from the engine. When buying old used engines that are not in running cars, I just look them over and make my best guess based on the asking price. Sometimes I'm lucky and sometimes not so much, but usually not much money is involved.