Yeah I'd jump on it, too. Most likely you're looking at a carb overhaul (there is an excellent step by step pictorial on this at the yahoo 400f forums, which I can sent you a link to), new tires probably and perhaps a cleanout of the gas tank. Then your normal tune up stuff like oil and filter, air filter, valve adjustment, timing, and cam chain. You may have to invest in some basic specialty tools (timing gun--about $40, impact driver, feeler gauges and common metric tools), but that should do to start with. You might also have to end up overhauling the front brakes, too, depending on what you find.
One note of caution on these bikes, and something you can certainly check before you buy: The cam chain tensioner mechanism on the 350 and 400/4 is not well designed, as the bolt that holds the tensioner in place can seize in the lower case. If you can get the owner to demonstrate that this bolt is not seized, that would be a major plus. All it involves is loosening the locknut and then (CAREFULLY) backing out the bolt. You want him to do it because if it is seized it can break off inside the cases rendering it almost impossible to fix without splitting the cases. If the bolt is free, you still need to do more digging to make sure that the tensioner still pivots inside the motor, but that is probably more than the owner is willing to do. But the main thing for now is that the bolt screws in and out of the cases freely. You'll need a small ratchet with an extension and a 10mm socket for this (and maybe a 12mm too? I don't recall...).
