I'm similar to the above.
I've got a 20hp 'stump humper' made by rhyscorp, an aussie company. It's about the best in its class in australia. Self propelled, centre pivot and come with a few engine options. I got the 20hp kohler, sometimes find myself wishing I'd gone with the 27hp. It's 29" wide. Goes most places and chews through stumps up to about 3' reasonably quick (20min-half an hour). I price it out at $200/hour, bigger stumps I sub out. Someone with an RG90 (access permitting) can smash out a big stump in 15 minutes that would take my stump humper 6 or 8 hours to do. If there's no access, then it's going to be a long day and a big pay check. But still not big enough.
Look for self propelled, narrow, at least 20hp, and centre pivot. Also look for a reputable engine with support. If you really need a tight access grinder, go for the alpine magnum (aka back breaker). Also worth looking into teeth systems that make the most of small hp engines like the multi tip system. I'm still up in the air over that one... the high buy in cost has been holding me back.
Shaun