The facilities you want on your property is really the only consideration for whether flat or hilly would work for you. If you mostly just want a pen for the horse and nothing else, hilly is fine. If you want a fed shed, arena, round pen, turn outs and a barn......flatter is better. My property is flat as a pancake and I wish I had a couple of hills, the horses will self exercise a bit better in hilly terrain.
If you could post a picture of the property showing the boundaries and the slope, you might get more useful answers.