swampy,
I've been using solely a vermeer 252 for 4 years now. After renting one for a year, I had no other option than to buy my own. I found that rental shops just don't keep sharp enough teeth on machines since most renters are homeowners who don't know how to properly operate a stump grinder.
I've ground probably a thousand stumps by now with my vermeer 252 and my only suggestion if you get one is to throw the vermeer teeth away and switch to greenteeth (which I use) or the sandvik wheel which I've heard a lot about. The greenteeth switch out so much faster than the factory teeth do. As a result, you beat up your machine far less by keeping it set up with sharp teeth all the time.
I just completed a 500-stump city contract that included stumps from 6-60" in diameter. I ground every single one with the vermeer 252. Big stumps do take longer since you have to clean up grindings as you go or else they pile up and block your view of the remaining stump. However, for just getting your feet wet in the business, I wouldn't hesitate to advise you to start with a machine of that size.
As for me, now that I've been in business for 4 years, I now have sufficient clientel to justify adding on a bigger grinder which is what I've just done. As part of a business buyout, I just acquired a vermeer 665A (65hp) grinder and a Rayco Super Jr 1625 (basically the same thing as my vermeer 252).
The 665A will hopefully help me tackle the larger stumps in less time; however, I'll still keep my 252 handy for backyard extractions and for when I'm working on plush lawns.
If you're interested in the Rayco 1625, I have more info on it listed in the Trading Post > stump machines section. It's a 1998 with a new motor and only 310 hours. It runs like new but, I'll sell it for half the price of new.