Although I do agree that cutting aluminum on a router is sub-optimal; I have seen people on YouTube do it and get results that were... eh... OK. The successfulness of the operation seemed to correlate with the rigidity of the machine; and taking light cuts was a requirement. Also, aluminum does vary - some of it is very easy to go thru; other grades not so much (this is as compared to hardwood - of course; compared to steel, all aluminum is like butter).
For working on aluminum, if you are going to get a mill and convert it (or perhaps even get an older VMC that has some life left in it; and modernize the control); I would look for a machine with a spindle speed on the higher end of the spectrum - you will get better surface finishes that way.
Do you have 3 phase power available to you, or just single phase? That's going to be the primary limiting factor on how big of a machine you can get; above a certain size the VFDs used to drive the spindle motor require 3 phase input (although some VFDs can be told to ignore the missing phase alarm and run anyway; and you just get a bigger VFD than the motor's KW rating would suggest).