Hi da and inv -
As Inveresk just proved, 31 diesel hp can drag some big logs (very nicely done). There is SO much that goes into a tractor-buying decision. Yes, there will be times when you had more weight and power, no doubt about it. I have a 5030 set up for tree work, and sometimes I wish that too. But if that size tractor will serve all of your other needs nicely, and you're not going into the logging business, I think you're on a good course.
For some ideas, maybe check out my post "5030 skidder etc" in Kubota Owning and Operating. I have a rear grapple that pics logs right up off the ground, but for skidding them I would recommend you look at one called the Beaver Squeezer. Forget the Iron & Oak grapple distributed through Bailey's, it's junk.
You should be able to find a lot of info on the Kubota site - it took me a while at first to find the grand L's, but there's a lot there.
And think hard about getting a hydro tranny. No shifting, incredible control under all conditions, infinite speed control, no slipping the clutch, easy to maneuver in tight spaces, easy to creep along with high heavy loads, it goes on and on. Unless you're going to be plowing fields all day long, the only good reason not to get it is if you just can't spend the money. But borrow it or steal it if you have to.
Anyway, good luck, have fun, and be careful!