Inca don't need mountains. but it's a good idea to settle near mountains to get lots of food from the Terrace Farms. As to whether Inca's UA or UI is more important, they're actually both incredible. Even if there are no hills on the map, Inca's UA reads at worst "pay only half cost for roads." I don't think I've ever had a bad start with Inca, even if it means I only found a couple of cities. This civ can easily go wide or build very tall if there are a few mountains.