If you're just wanting to machine aluminium, then a small well built router style machine may be a better option, in which case I'll defer to those on here who know far more about them.

However, if you want to do tougher materials, then retrofitting a desktop mill may be a better option. In which case, given the working area you've quoted, I'd look into something X2 sized. There are quite a few options, but if you have a search over on the desktop mill/(S)X2 board on cnczone, then you'll find plenty guides. Also, Hoss over there sells conversion plans, which would remove the need for a lot of head scratching.
Another option may be something like a Denford Novamill, but for that working depth you'd most likely have to add a column raising block (there are plans over on the denford forums)