This topic has been discussed here and on the other reloading sites. I have never seen anyone write that buying a Dillon was a mistake. I have had one since 1995 and use it more than any of my other presses.
A guy I know who deals in used presses. He almost never gets one to sell, and when he does, it goes for big bucks. They only get sold when the owner dies.
Comparing a Dillon to a Lee is like comparing a Cadillac to a Yugo. A Dillon will almost never go bad, and if they do, Dillon will fix it for free.
They sell the 550 for $440 w/ a conversion kit but w/o dies. Brian Enos may sell them for a bit less. You don't need to use Dillon dies w/ the press; use whatever you have. You DO need one of their powder dies as the powder measure attaches to it.
As for how much it costs to setup the machine for a second caliber, it depends on how much you want to spend. If you want a complete second setup w/ die plate, conversion kit, powder measure, and stand, it will cost $150 w/o dies.