Actually, the $13k price kind of reflects that the unit is a mongrel. A 2,000+ hour B21 with the correct backhoe can go for $16-20k. A 700 hour one would go for more, most likely. They do offer the B2x series as a tractor-loader only, so the lack of a factory BH isn't especially odd, but you might suspect someone decided to add the hoe to such a unit and didn't want to ante up the $8k or so for the correct one from the factory. Instead, they found something on fleabay or wherever and made it fit.
The 4520 is actually an OK backhoe and one in good condition should do a lot of work. It all gets to what condition it's in and how well it was installed on the tractor. Kind of like buying a car that someone dropped a different make of engine into. Could it work? Sure. Would it be a sound installation? Depends on how it was done. Is it worth the same as a fully stock setup with the right motor in it? Not really. Final thought on the 4520... best I know it had a 2 stick control layout. That photo looks like there are 4 separate sticks. Is that right or am I seeing things?
If the tractor and loader are truly at 700 well-maintained hours, you might figure it alone could be worth most of the $13k. Suggest you track down a complete, working B21 with backhoe and test run both machines for comparison before making any offers on this one.
Please recognize I don't have a B21, so this is just general advice. There are quite a number of B2x owners around who may have better, more specific comments.