You should probably study up on clutches so you know what that might end up costing ya:
http://www.google.com/search?source...UTF-8&rlz=1T4DMUS_enUS221US222&q=kt100+clutch
Unless everything looks a lot better than his pics, I don't think I'd offer over about $300 to $400 for everything. He was wrong about the chassis brand, so I'm not sure if I'd trust him saying the engines (or anything else) are in good condition. It'd be a bummer to have to spend more money freshening an engine up right away on top of the clutch and brake issues.
Looks like the rear of the frame is super rusty, like maybe it sat outside or something. That's not a great thing for the rear axle bearings, make sure you give the rear axle a spin. It should spin smoothly and not have any exsesive up/down or side to side play.
He didn't picture the front bumper, but if its all bent up just keep in mind they don't come cheap to replace. You could always make one but sometimes they're difficult to find new for some kart brands.
Are you thinking about racing this kart at a track somewhere or just some neighborhood/parking lot cruising for now?