First, does the odometer work ?
Good chance it isn't, as this one of the problems with these bikes, if it doesn't work, use the unknown mileage as a bargaining factor .
Obviously, tire condition, check for date of manufacture on the side wall, it should be a 4 digit number, like 4608, which would mean the tire was manufactured in the 46th week of 2008, just because a tire looks new, doesn't mean it isn't old, and old tires are not your friend .
If it has rubber brake lines, good chance they are original, and should be replaced .
Fork seals for leaks .
With the bike on the center stand, grab the front wheel and try to move it front to back, checking for loose steering head bearings .
Check for oil leaks on the engine, with it running .
Check throttle and 'choke' controls for smooth movement .
Transmission neutral switch for fluid leakage .
Check the rubber boots on the transmission for leakage .
Check battery for condition, if it's a serviceable type, check for fluid level, corrosion .
If the shock absorbers have an adjustment lever on them, they are probably original, and should be replaced .
Final drive leakage, you mentioned fluid coming out of the breather, it could be over serviced, a common source of leakage here, is the brake cam shaft o-rings, if they are leaking, there should be oil seeping out between the wheel rim, and final drive housing, and if it is, a good chance the rear brake shoes are oil contaminated .
Check the control switches on the handle bars, the turn directional and headlight switches seem to be the ones that fail, check them for positive detents when moved, if they feel 'limp' they probably are close to needing replacement .
The fuel filler cap, if it's been over tightened, it can start spinning when turned, and not come loose .
Check the interior of the fuel tank, for rust, and condition of the liner .
False neutrals, can be a shifting linkage issue, or operator technique .
