What year and him many miles/hours? Looks good far as I can tell, that Supermoto setup is worth around $800 give or take. The bike itself without the Supermoto parts is probably worth around $2500-3000 imo, more if it is a later model.
WR's are good bikes, but they are a bit heavier than some other 450's and they feel like they carry their weight up high, which means they can be tippy in the slow moving, technical riding. They are very reliable, good trail bikes and and don't require an excessive amount of maintenance.
The WR450 is pretty powerful, it will pull the front tire off the ground pretty easy. It's very manageable power though, just be aware that it will go when you rip open the throttle!
I love my '04 WR450 for desert and trail riding, its a fun bike.