Used or new? Big power head with a long bar to cut stumps down as far as you can.?
Think about it we can’t always get a stump grinder to the stump if the ground is uneven. I’m not sure what you want.
I would get the biggest power head I could get my hands on with a 36in. Bar. Also, I would buy a little used saw for cutting roots, and definitely a blower for the stump grinder air filter and blowing stumps prior to cutting low and mulch after the grind.