Yes, I would say you'd want a surface cleaner if you plan on doing more than the occasional driveway...You'll get better results, and you'll get them done much faster...Whether you're doing them yourself or having your helper do them, time is money...It's much harder to get a nice consistently clean driveway using a wand....You can get a cheaper 16 or 20 inch hover type for around $400.00...that'll pay for itself the first week if you're doing a lot of driveways. The surface cleaner allows me to clean concrete at least twice as fast as with a wand.
I don't know that a 2gpm machine is going to be adequate for doing concrete with any speed, and certainly won't be adequate to clean with a surfacer.
I agree with Mike on the Insurance...Unless you're already insured to do pressure washing, you likely wouldn't be covered if you have a claim. I'm insured to do Pressure Washing, but if I decided to do lawn care also, I wouldn't be covered without adding that to my policy.