Im no athlete, but find that a 28/28 can get me up some extremely steep hills with no problem. For your very first ride you may find it hard, but after 3 weeks, your climbing muscles will have adapted. If you want to carry touring luggage up a hill, then you may need something lower.
Bikes differ in their absorption, but you generally get used to what you ride. I ride a non-sus road bike on trails and off-road, more extreme than 90% of MTBs ever see.
Fit is , as you understand, very importantt. Also, make sure that there is tyre clearance at the brakes for sufficiently wide tyres, and fenders if you ever need them. Heel clearance for fenders is about chainstay length, which is easy to measure. 42-43cm is enough for panniers, esp with a heel cutout, and if your shoe size is quite small.