I second this, definitely make sure all the rubber is in good shape. My K100 I fixed up this year had been sitting since 95, so all the fuel lines were shot. Injectors were clogged, fuel lines in the gas tank had deteriorated into black goo, and all the vacuum caps on each throttle body were cracked. I replaced all of that, and it runs great now.
Make sure tires are in good shape of course, if you have the chance you can lube the splines too. Other than that, I recommend changing all the fluids if they're old. Coolant (and a new thermostat is a great idea, those springs wear with age and you don't want that failing, lest overheats), oil and filter change, brake fluid flush, and transmission/final drive oil. These are all pretty straightforward procedures, but important nonetheless.
Let us know if you have any questions on fixing her up, also, the odometers might need some new gears and some tinkering in case they're not working.
