Just adding to what txdon said, most people say that a subframe type of backhoe is actually easier to put on/take off than the 3pt variety, which is sort of contrary to what one might assume, considering how easy most other 3pt implements go on and off.
What's different, I guess, is that a backhoe must be REALLY attached to the tractor, because the forces involved in using a backhoe (i.e. digging and prying like there's no tomorrow /forums/images/graemlins/grin.gif) are much more stressful than just lifting a 600lb brush mower. The 3pt variety of BH is really only good for so much of that; the subframe type is much better. So, I think it's possible to buy a larger bh with 3pt mounting, but that's sort of asking for trouble, in my opinon.
Since you are doing this for a friend (where does one sign up for friends like you guys?), and if time allows, I'd suggest you cruise around Ebay to see if something comes up. In my area (mid-Atlantic states) there is a bi-weekly Heavy Equipment Trader publication that sometimes has such deals also.
This could turn into something of a quest, but you will have something of much greater utility if you come across the right item. The factory backhoe for an MX5000 is the model 4560 mentioned in one of the other replies. It has its own PTO-driven pump and reservoir, so it does not need to be plumbed into the tractor hydraulics. I have that same backhoe on my tractor... it's a very sturdy, capable unit. If you can get a good used one for the price the other fella mentioned, jump on it!
Other comparable models to look for would be Bradco 509, Woods 9000, Bush Hog and Rhino (don't know specific models). Remember, you'd probably have another $1-2k involved in getting the right subframe, etc. besides just the hoe itself. They could be self-contained like the 4560, but it's more likely they would be set up to run off the tractor hydraulics (something to be sure you find out when buying used!).