All of the things previously mentioned and a few more. You'll want to note whether it has the distributor at the front of the engine (between the block and the fan) or out the side of the block near the front...nothing wrong with either, but the side distributor models are later, and have a bit more power. Check for play in the front end/steering....many are so loose the front wheels flop left and right while you're driving straight. The really big thing is to check the oil pressure when it's at operating temperature. The 8N never had a great oil pump, and with a little bit of wear, can barely put out enough pressure to keep things working. Warm, at idle, you should still see at least something like 15PSI, and probably 30-40 at anything significantly over idle.....any less and it means the pump and/or bearings are worn out.
Also, keep in mind that there are a lot of Franken-8s out there that are part 8N, part 9N....it should have a 4spd transmission, and rear wheels that are one-piece where the 9N is 3spd and two-piece rear wheels. I'd also make sure to look on the inside of the rear wheels to see if there is any evidence of oil leaking....not an expensive fix, but something he can use for negotiating the price.