From the pictures:
1. Rear fender and mud flap is not stock. Make sure the frame wasn't cut and/or hacked up to accommodate the new fender. Also make sure the turnsignal tabs are still in place. Wiring will also have been mess with. Unfortunately most DIY'ers use junk crimp connectors from the auto parts store and electrical tape so these amateur at best.
2. Side panels are missing. Cost to replace if you can find used is $100+. Side panels are a necessity IMHO as they protect some electrical components.
3. Front fender has been changes out. Not a big deal.
4. Motor looks clean. Have the seller take pictures of the underside of the frame and motor to verify there is no dented frame tubes and/or repaired holes in the cases.
5. Tell him the seat cover he can keep. There is a 99.9% chance your going to change the seat out anyways and the stock seat foam and cover are just about worthless. Seat concepts make several seat variants for the DR. You get new foam and a cover and reuse your seat pan. A hand held stapler and 20 minutes and you have a seat WAYYYYY more comfortable than stock.
6. New tires are nice but if you can stop him now from buying and take that off the cost of the bike do it. Tires are personal preference. What he is putting on you may not like. When buying a used bike I'd rather the tires be work out and get the bike at a reduce cost. Then I can put on the tires I like. Honestly for the DR the tires I like best are Shinko 244's. A full set costs $100 shipped to your door.
7. The exhaust is dodgy at best.
8. If you go to see the bike take the headlight cowl off and look to make sure the wiring under there has not been butchered.
So if you can stop the seat cover and tire take $200 straight off the asking price and negotiate from there. I'd shoot for $1,500 and pay up $1,700ish. If the frame is dented underneath or has been modified the offer price would drop into the $1,200 range. Again there are no shortage of these bikes so if your gut feeling is walk away.....walk away.
Also if your not a member join up on ADV Rider lots of bikes listed there f/s. No matter what you do, do not panic and buy a KLR