I second the 35HP tractor. The extra weight will more than outweigh [no pun intended] any loss of maneuverability. I mow 6 1/2 acres with 6' RFM on a 39HP, dodging about thirty trees. The extra time it takes to do the trimming is made up for faster mowing. I also have a B7500, and it is a good tractor. But there are lots of jobs that is is just too light for. If you can only buy one tractor, buy the bigger one.
Do you plan to chainsaw the trees, then bring them out? Or are you wanting to take them up with the tractor?
You want to spread the wheels out as far as you can. And if you have the FEL, keep it low to the ground. Most all 35-40HP tractors will have a pretty low COG.