Stihl or Husquvarna. I used to work in forestry years back and used both these, I wouldn't recommend any other make.
All chainsaws have the same sharpening system, surely, don't they?
The shorter bar you can get away with the better. A 13" bar should manage a 24"tree no problem.
Big bar need more power (heavier saw).
I use Husky and prefer them over stihl, but both make great professional saws.
P