I had an 04 250f, it was a good bike but had some first year issues, mostly valve problems, or more precisely the valve springs were to weak. I would think the previous owner would have had that problem taken care of by now. As to your oil filter issue, in stock form you did have to drain the coolant to change the filter, they changed the design in 05, you can purchase a aftermarket coolant cover/oil filter cover, in fact I've seen many on ebay. I wouldn't let that bother me, maybe it's been changed already, or you can change it.
Always keep a clean airfilter, change the oil, and most of all make sure the valves are adjusted properly. If you follow those three simple things it will run for along time.
Good luck
Don