In my opinion, that's too much. $1500 would be a good price if it was in really good shape and didn't have to fix anything. However, you have rust, paint, body work, and interior issues. Make a list of what needs to be replaced and call around. See how much more than $1500 it's gonna cost you to get everything fixed. If the mechanic says it is mechanically sound and you don't care about fixing the aesthetic problems, then I say buy it and drive it until it falls apart.
The rust concerns me more than anything at this point. It eats away at the metal like cancer and you don't always see it until you start digging. Make sure you do a thorough inspection under the hood: battery tray, strut towers, radiator support, firewall.
I'm not trying to be negative, I just want to make sure you get a good deal and not taken advantage of. We've seen a lot of people pick up a great car for a smoking hot price, only to get it home and realize they bought someone else's headache.
Good luck and let us know what the mechanic says.