If it starts when cold at 18F the compression is just fine. You don't need a compression check. Cold diesels with low compression have trouble starting in cold temperatures. Just make sure when you go test it again OR buy it, that its absolutely bone cold and then check the starting again. The guy could have started it before you showed up to test drive it. Was he using a block heater before you arrived? I'm not a trusting soul.....
Good compression, no rust, $1,000? What are you doing sitting on your rear reading this, go pick it up.