Welcome to the forum, see if there is any play in the rear wheel or leakage around the final drive, that may be an indicator of an FD failure. My understanding is that FD failures are about 5%-ish, some of the guys might be able to correct me so don't stress over it, it's fairly rare.
Make sure the ABS works, take a look through the sight glass on the brake reservoirs (on right hand grip and open right saddlebag there is a little window). If the fluid looks clean the guy has taken care of his bike - at least one of the more sensitive components. When you get on the bike for a ride and roll about the first 10 feet you should hear a sound like a latch engaging and the ABS lights should go off, they will flash until you start moving.
The shift hiccup could be an indication that the shift linkage may be going bad, don't know much about this particular issue but I know that it is out there. Again, some of the other guys might chime in on that or do a little searching on the site.
Finally, do some homework on your own and visit the halls of wisdom. Make sure it looks like it is well cared for and the guy seems reasonable trustworthy of course, and make sure the tires are good, and not too old (last 4 digits of DOT# indicate week and year of manufacture). The rear should also have 'REINF' or 'REINFORCED" on the sidewall. Tires are a bit pricey for these beasts so if they need replacing make sure you nudge him down on the price to compensate.
Be ready for the weight but not afraid, and be ready to fall in love with it the second you pull away on that first ride.
Edit: Can you link the ad for the bike and or pictures?