My limited experience has taught me that with tractors, HP is really only applicable on the size of implement you're able to run with your PTO. Every tractor I've ever seen has been able to produce enough torque to break traction in any reasonable gear regardless of how its weighted.
My 1983 2wd Ford 1710 with its worn out 770B loader, wheel weights, and weighted tires runs around 4000 lbs. It will drag 2 or 3 16" logs that are 16' long with no problems. I'm only dragging them across level ground. I normally skid them in either 2nd range 3rd gear, or 3rd range 1st gear (12spd geared transmission). I'd estimate that I'm moving around 4 or 5mph. Having said all of that, my F350 (4wd, crew cab, long bed) skids logs out to my trailer a LOT faster than a tractor will so long as the ground isn't too soft.
If you're trying to pull out hung tree, even a 35hp tractor is going to struggle. I've managed to yank them out with my tractor (very hard on the tractor). It's been easier to yank them out with the truck. Best way for me has been to put a 12K winch in the trucks rear receiver and use that to pull them out. I could probably put it and a battery on my 3pt, and chain the loader to a tree, and do the same thing, but its easier for me to use my truck since it skids better anyway and the logging roads on the property can get me to the general area.
As far as lifting logs with a grapple is concerned, I think you're going to struggle to lift a 16" x 30' log with a grapple on a 25hp tractor. You might be able to lift it up, but even if your machine weights 4000 lbs, you're not going to have much traction.
All of this to say, get the heaviest tractor you can with the most 3pt lift capacity and the most loader capacity. If you still have money left in the budget, then buy more HP.
