You'll have to stick with whatever dealers you have nearby.  I'd look at local hay auction results to see what sized bales brought the most money and what the popular size is.  You might find everyone makes 4x4 bales and save a bunch of money not needing to buy a 5x6 baler.  Or maybe you can create an opportunity by being the only guy in the area with "X" size baler, lot more risk in that venture.

4x6 bales tip over very easily, 5x6 are harder to ship but a 5x6 can make 5x5 that ship easily.  We have a 4x6 (JD466) and a 5x6 (Vermeer 605SM), the 4x6 does all the distant jobs that we haul hay back from, usually 4x5.5 bales.  The 5x6 does everything on the place and usually full size, although it is nice to make some 5x4's every now and then for those places that won't need full size bales fed.

Hard to say on number of bales, just so many variables on what was baled and how the machine was maintained.  Make sure to have a water fire extinguisher mounted to whatever you get.  $150 extinguisher can save a $30K machine in a hurry.