These are very reliable bikes. At 20k, the chain & sprockets are either due for replacement or just replaced. Factor that into the price, about $200 DIY. Check whether all the fluids have been recently replaced and the valve clearance checked. Everything else just takes a good look-over and test ride.
+1, check to see if he's a one owner, why he's selling. Many wee owners put numerous extras on after purchase, so look for extras. Also be aware that withthe new release of the 2012 wee, older models may tendto be worth less than current book values as seen with other vehicles, weather or not its worth waiting is your call as the amount of depceciation may be negliable, Also keep in mind the season, winter will bring you lower prices, although the season is waining.
See if he maintained any type of service records, this is an invaluable item when selling, and typically he can stand firm on a fair price.
I think there where very few changes after 04,mostly emissions, so keep an eye out for other years as well with less than normal milage for your area, also be aware of factory options that may be handy for you, such as factory heated grips( hard to beat these with any aftermarket) and center stands..
Good luck and remember....the patient buyer always find the deals.
