Its all about the HP. I was aware of this but didn't experience it until my purchase of a new Vermeer SC652. Easily 2-3x faster than my SC352 with a 34 HP Diesel but, also 2x the payment and 2x the cost of insuring it for theft/damage.
When I started doing this part time 7 years ago, I had a lot of concern about having enough work to cover expenses associated with this venture. Thus I got in with a more affordable 2002 SC352. I got a used one with 89hrs on the engine and cost me $330/month on a 5 year plan. I traded the 2002 in at 1100hrs with a blown engine for a 2009 SC352 Diesel with 54 hrs. The 2009 was costing me $430 a month over 36 months. It wasn't until two years ago that I could afford to give up bartending part-time to focus on my business. Last year I was financially comfortable enough to upgrade to a larger HP machine. I tried to add 2-3 new referral/ sub-contract opportunities a year. You need to build a reliable network of companies to sub out to. One is not enough. When the tree guys are slow, your business is dead.
I did a couple thousand stumps with my SC352's. Lots of large land-clearing jobs with 75 or more stumps. Was generally happy with them until maintenance issues started to become a quarterly routine after 500hrs. The smaller grinders can't take the pounding like a larger one. That said, I would start small (min 34HP), build your book of business and, when the numbers make sense, upgrade to something with 60HP or greater.
Other things to consider....if not too late
1) Get a machine with 4WD! Too much time wasted trying to push machine up a hill side plus when going up an incline you will be less likely to cause any turf damage. Also will make it easier to go forward, up and over the pile of grindings you create.
2) Get a push blade. Makes back filling holes much easier. With a smaller grinder you will need to push/move the debris around. Especially when on larger stumps. You will need to re-position the grinder and to expose any areas of a stump you missed.
3) With a larger HP grinder also comes a greater sweep radius. This adds to the speed with which you grind. You won't have to reposition the grinder when on a 50" stump.
4) When the time comes, and you can afford it, GET A REMOTE CONTROL!! IT IS AWESOME!! Better for your hearing and breathing and you can do the grinding from the cab of your truck when its raining or freezing out.
5) Have some door hangers and business cards printed and when you are on a job-site look in the yards of the neighbors. I do the property to the right, left and behind. Its easy foot canvasing.
Good luck!