I've owned a Gravely of one sort or another since 1984 - an old Model L. My current Gravelys are both "newer" (still late '70s, mid '80s models) with Kohlers on them. I prefer the Kohlers just because those engines are what I'm used to working on. The older Gravelys use magnetos instead of points/condenser. If it's a manual start machine, it's done with a strap. If you're comfortable working on stuff like that, the older ones are tough old machines.
For mowing on the hill, make sure you get dual wheels. Up until 9 months ago, I'd never used a Gravely with a steering brake, but I can see how it would be super handy going across a hill side. Get something with the 4 speed transmission.
My machines are 10hp and 12hp. I've never owned an 8hp Gravely. I might think that it would be a bit underpowered, but these things are geared pretty well so you might be OK. But, I'd opt for at least a 10hp if you can find one you like.
Something that I want to eventually do is buy an older Gravely and then repower it. There's a guy named Bruce Guthrie that makes the appropriate adapter plates to take an old Gravely where the engine and tranny were integrated and then remove the engine and replace it with a Honda clone from somewhere like Harbor Freight.
IndustrialToys mentions that a Gravely is "cumbersome". He is right - they will work you to death! When I lived in a neighborhood, several of the neighborhood men asked me to try the tractor when I was cutting my lawn with it. It was always fun to watch them struggle to control it! Once you get the hang of it, you can handle it pretty well. I wouldn't be without one, regardless of the other tractors I might have.
Good luck!