Id ask the seller why the car is Cat D registered, then do a vehicle check to make sure the story they give you checks out. Insurance wont be a problem. Tell them just so youve covered your back and only insure it for what you paid for it. They wont charge you any more, and any main insurer will cover a Cat D. Apparently 1 in 15 cars on the roads today is Cat D registered - I bet most of the people driving them dont know it either!
As for price, I reckon it looks about right. Maybe try and get another couple of hundred off for the highish miles (average miles for age should be more like 30k), but if youre happy with it, then go for it.