If you are going to do small areas and not expand into 10,000 s/f or bigger a smaller surface cleaner like the one pictured is fine. The more flow (gpm) you use the less likelyhood of striping the concrete. You'll also be able to move faster. Concrete pros use 28"+ surface cleaners with machines ranging from 5.6 gpm (too small for a surface cleaner that size, in my opinion) to 10 and 12 gpm machines for faster cleaning. If you use the right cleaning agents/chems, you shouldn't need more than 3000 psi. A semi decent concrete setup for medium size (0-20,000 s/f) cleaning duty would be a 28" Big Guy on at least a 5.6 gpm hot water machine. With hoses, reels, the right guns for downstreaming chemicals and the above cleaner and machine you are looking at around $8000.
You do realize there are federal reclaim laws for cleaning concrete, yes? You have to make sure you either recapture water, divert it to grassy areas or send it to the right type of drain. If you get caught sending wash waster (even if you don't use chemicals) to a storm drain you can get popped for a $10,000 fine. Reclaim equipment can set you back another few grand.
You can make decent money doing commercial concrete but you have to be fast at it. Here's a couple of pics and a link to http://www.pressuretek.com/zxzxzxzx.html