By the time you pay it off, it could be time for a new one, so maybe if you can find a really good deal on a dealer refurbished one. A tree co I worked for got a dealer refurbished 15" chipper for what I thought was a very good price, and it worked well too. The chipper we had been using had a fraction of the hours, but reached a point of breaking down all the time. Strangely the dealer seemed to have a hard time getting it back to 100%, so we got the refurbished one.
You might just wait until you have enough stumps to grind to make renting a good big grinder worth it for a day or 2. Then maintenance and total machine cost is not your problem, and you can plan to use a large grinder only when you know it profits you. You'd just have to check with the customers to see if they are ok with a 2-8 week waiting period as you get enough stumps on the list. Stumps aren't normally as urgent to get rid of as the tree is, and it can give the customer a chance to save up a little for the grinding.