We've got 20 acres of mostly pine forest with a little bit of natural pasture (maybe as much as 5 acres but I've never measured and guess more like 2 or 3) and are in the process of building a cabin and orchard.
Since the land is in the hills (and more or less a hill) I have some grades between benches to deal with as well as 2+ feet of snow in the winter. We don't live on the land but would like to make sure we have access year round so we'll have to plow/blow snow during the few months of snow in the winter.
I also plan to clean up the access road and work on some roads on the property as well as clear brush, mow, move slash piles to safe areas for burning, clear rocks, mow brush, bushes, saplings to increase meadow area and disk/plow/till the acreage that we have that's meadow so we can start growing some cover crops (to fix the soil for other crops or perhaps just to attract game).
At some point we'll likely put a few cows on the property to eat and sell annually but I have a fence to build first among other things.
I also have a small sawmill (Woodmizer LT10) and would like a tractor that I can use to pull logs to the mill as well as load them.
My father who lived on a farm when he was a boy (born and raised actually) tells me that 30hp is about all I should need. His dad had a Farmall A that was about 15hp so perhaps he's right...or not. Anyway, I'm thinking 30-35hp and 4wd but all of that is just from tire kicking, talking to salesmen and gleaning info off others on the net.