One thing you ought to know is that the Yamahammer is a Non-interference engine. That means that the valves are never SUPPOSED (under normal conditions) to contact the pistons, even in the event of a broken timing belt. This is quite different from most all Mitsubishis, Chryslers, Hondas, Acuras, and GMs with Timing Belts. All that means is that if your belt breaks you tow the car home, line up the marks and install a new belt. Problem solved, assuming that you're dealing with a stock engine. Depending on how many miles are on the car, if it's anywhere near a 60K interval I personally would want to just save my money and do the Full 60K including the valve adjustment. If you go ahead and do the timing belt, since you know nothing about its history, you may want to do everything just for peace of mind.
BTW, my dad and I had never put a timing belt on anything until I got my SHO. We put in a new belt and WP and 15K later I haven't had any problems. If you have a pretty good mechanical inclinaton and are willing to possibly have the car on jackstands for a couple of days, I would say you can most likely do it yourself, save yourself the labor costs, and have the pride of knowing that you did it yourself.