This is a question with a complex answer that could fill a book... Yes, 10W can do it, no problem. Everyone will tell you it's impossible, especially emcomm people. The antenna and the way it is installed makes all the difference. Now, apprarently you are into emcomm, not survival radio... Big difference.
100km is a difficult range to cover. I would say NVIS on 80m and VHF SSB with a Yagi-Uda antenna. Between the two you should be ok, but don't expect 95% reliability. It's radio, not land-line telephone. You can't even throw numbers like 95, 99%, those are meaningless. 99% when? Where? How? Sometimes you'll get 100%, sometimes you'll get zero, as fancy as your equipment might be. Practice will teach you how to increase your reliability, not equipment.
In your case being on a hill is the key. You can go very far, even 100km with a 5W handheld VHF radio if you are high enough. A friend on mine made a 200-mile contact using a Yaesu VX1 with an output (using AA cells) of 100mW. We were at 2000m altitude near the sea.
Also look at the JS8 digital mode using the JS8Call software.