I think 10.5 K is a pretty good price for a B21 with only 250 hours. I would look it over pretty closely, checking for leaks underneath the tractor and at the hoses. If you have a friend thats experienced with tractors, bring him along and see what he thinks.
Things to look at and for would include; tires, battery, radiator fluid, hydraulic fluid, bent cyclinder rods, and general overall condition of the unit.
In my opinion,the B21 is a dirt moving machine. It's pretty compact, stable, well built, easy on fuel, and is a bulldog in the dirt.
You haven't said what your main usage would be. If you're looking for a dirt mover, the B21 is great! The backhoe is easily removed for access to the 3PH and PTO. That being said, the PTO HP is fairly low, (around 13 HP), so if you're looking for 3PH stuff, you are somewhat limited in size for accessories.
I bought a used 2004 B21 about a year ago, and I'm very happy with it.