I'd stay away from the 500. The least expensive one would probably be a used 2000 classic, which you can get for about 400 bucks give or take a few. The 2000 aside from having less sample time than the nwer mpc's is still a very capable mpc.
The best " bang for the buck" option would be a used mpc 1000 with the JJ OS upgrade.
Again if money isn't an issue I'd go with the 2500.
There isn't too much of a difference in terms of the learning curves to using an mpc, with the 4000 being an exception. If you can use a 2000 you can use an XL, you can use a 1000 you can use a 2500. They feel a little different but the idea is the same. The 2500 is a well rounded machine if you have the dough and once you get the hang of it you can dig deeper into it. If its about cash go with the 1000.