I'd lean heavily toward listening to markyscott, in that unless you have a LOT of large deadwood to carve, your dollars might be better spent on other tools than large die grinders... even a used Dremel will at least allow you to get a feel for what is "needed". Most trees aren't large enough to accommodate a 12" tool and obviously, the maneuverability of a larger tool restricts it's usefulness if there is the least bit of branch structure to work around.
Then, the bits aren't cheap either, I just happen to have a variety of them here from industry.
Like someone else pointed out, if you make your living as a pianist of as a guitar player, I'd stick with the manual choices... hospital bills to replace ground meat near the extremities come with a cost too... and a bit more pain than one would hope for when first heading toward that tree after start-up.
The problem that occurred within the used tool place started when I went into the place one summer's day and the RCMP were talking to the owner asking for receipts from the original owners of the tools he had on display (because many of them were engraved for ID). I'd say within 6 months, he was out of business, so obviously he was a fence for the kids who'd break into garages, looking for a quick buck. If this possibility bothers you, Kijiji might be the better option.