Welcome to TBN. Yes, the slope vs degree thing is something we've all seen before on TBN. Basically "slope" is expressed in percent rather than degrees, and it turns out that slope is roughly twice whatever the degrees are.
So your hill might measure 17 or 18 degrees and that would be a approximently a 35% slope. (45 degrees is 100 %).
No problem; us tractor guys on hilly land deal with that definition, and with that much slope, most every day.
Everyone is saying the same thing to your initial post: 25HP sure beats no tractor at all, so it that is what the budget will afford, then do it. But we also know that for what you are describing you'd be better off with a larger tractor - something in the 3500 to 4000 lb range. You can save money by getting a gear drive rather than hydrostat. You'll definitely need a loader, and the lower cost of the gear drive transmission just might pay for one. As far as implements that fit on the 3pt hitch - like a back blade or/or box blade, mower, post hole drill....etc. There's no need to buy those new. Used 3pt implements work as good as new ones, they just look used.
Tractors last a lifetime and you can sometimes find good ones used. Read through back threads on TBN and you'll probably find that the normal reason a tractor is for sale is that the owner wants a similar tractor in the next larger size and maybe some specialized options. Quality used tractors can give you a lot of machine for your money. Is there any reason not to consider used?
rScotty
Don稚 have a problem with used. After driving a few I know i want HST. Used ones I see are around 15000. That痴 one reason I thought might as well go new to get a warranty.