Have you got any other power tools? if you do, it might be worth getting that brand so you can use the same batteries. If not, go for a flex cordless machine. Probably the best cordless machines on the market right now for all round stuff.
Depends what kind of work you are doing if it will warrant a cordless version, mobile detailing is much easier with cordless kit but anyone working in a unit or just a weekend warrior is better off saving the cash and going corded.
If you arent using it often, get one of the DAS models. You can fit 5 and 3 inch backing plates on certain ones too. Spend the cash you save on some decent pads and polishes.