I wonder about the skidding timber necessity. If you are cutting for firewood, there is really no reason to skid the logs, just cut them to stove lengths where they fall and use your tractor bucket like a power wheelbarrow. If you want to bring the logs to a sawmill for making boards, then you would need to skid them.
For access in a tight woods, plan your harvest to create pathways, selectively cutting the worst and ugliest trees for firewood. Given time, you will develop a running inventory of trees to be cut in your head.
A good tree, healthy crown with a straight stem, is one that has the genetics to take advantage of a good growing site or conditions. Those trees are the ones you want to keep as seed producers. If part of your goal is wildlife conservation, then paying attention to species that are good food sources is important too. White oak, for example is a premier tree for deer.
I think you can do what you need to do with a 40-50 hp tractor. I've done a lot of those things with my 40 hp NH Boomer. That size class is a reasonable trade-off between costs and the ability to do what you want to do.
It's good to keep in mind that tractors are primarily designed for field and barn work, not woods and excavation work. Tractors will do those things, but not as well or as long as machines designed for those purposes will. The advantage of a larger, heavier tractor is it will take more punishment and you won't be working it to within an inch of, or beyond, its capabilities.