Look for rust on the exhaust (underneath, side, etc...) Look for holes, bubbles, etc...If they look good, that is a plus. Those 4-4 are worth some money.
Look inside the tank for rust. If there is there will be some work to do there. Open up the master cylinder, or have him do it...be sure to not get brake fluid on the paint it will ruin it. Make sure the fluid is clear and clean, if it's not that will require attention too.
I would offer 800 myself, and work up to 900 or 950 tops, but that's me. Tires are going to run about 150 with tubes, and rim strips, plus mounting/balancing (with you removing wheels yourself should be 20-30 each). If the tank is rusty, or the brake fluid is nasty, I would knock off 50 to 100 for each problem from that offer. The spokes look rusted a bit, so expect to spend some time cleaning them but that is another issue to talk him down with.
The paint looks good from the pics, if it runs good, go for it.
Good luck