Parts are no problem. Virtually all parts are still available OEM, and many are available aftermarket as well.
Reliability is more a function of the condition of the bike, how well maintained it is, and how many hours it has got on major components like the crankshaft, than anything else. I don't know of any major failure prone areas on them, but being an older bike, the more wear and tear it has without adequate maintenance and rebuilding, the less reliable it will likely be.
In terms of checking the bike over, general condition is a big one. If it looks like it's been well maintained, that is a good sign, but don't confuse a bike that has been cleaned up to sell it with a well maintained bike. Look at the chain and sprockets for wear, brake pads, leaks and slop in the suspension, pull the magneto cover off and check the main bearings for play, check the wheel bearings for slop, pull the plug out and look at it. It won't tell you everything, but can let you know if a failure is imminent. Pop the seat and check the air filter. If it is dirty, and looks like it hasn't been cleaned, the engine might have sucked in dirt. Slop in the controls, broken parts, and other things that don't look right all give clues as to how it has been treated. These are all standard things to check, and I'm sure I've forgotten a bunch. Don't be afraid to walk away or tell the guy if you've found problems. If the problems are minor, take try and get the price lowered if they are major, walk away and find something else to buy.