2. Mountain Lion needs 2GB to run well and with only 2GB it's really not that snappy. With one or two more OS updates it's going to be brutal on 2GB. So apps aside there is a limitation to buying a year old machine. What is made to be quick and current for 3-4 years will now be 2-3 for you.
False. I've been using mine for 6 months. Mountain Lion is extremely snappy, whereas my sister has a 2009 MBP C2D with 8GB RAM (the other machine the OP is considering) and it does not run SL smoothly at times. Especially the animations/ transitions. As far your second sentiment goes about the next OS releases, that could be true, but is the OP actually planning on keeping his for 3 years? Many people, especially on this forum, upgrade after a year or two anyway.
If it were me I would get this find an older copy of Snow Leopard and do a clean install to that. SL would run fantastic on that machine and as long as the owner is OK with rocking an older OS it would make a great machine.
I don't think this possible. You can't install SL on a machine that comes pre-bundled with Lion or ML. At least that's what Apple says. And why would he want to do that? It was MADE to run with Lion. It is fully capable of running Lion AND Mountain Lion. Do you own this machine? You are doing an awful lot of assuming for someone who doesn't own this model.