Yes. Height matters a lot, especially in a glass tank, because more height = more heat rising away from where the snake spends their time. For certain snakes, like tree boas, carpet pythons and even corn snakes, you want some height because they climb. For Ball Pythons, however, you need more floor space and little height since they are burrow dwellers.

36x12x18 should be just fine, but ideally the 12 and 18 would be switched so the width was 18 and the height 12, but those extra 6 inches shouldn't hurt much if you make sure the tank is insulated and the screen top foil treated.

And yeah, I know what you're saying about the 'reptile tanks' vs aquariums costs. I have 4 tanks working right now and only 2 are the 'made for reptiles' kind. Both work just fine as long as you prepare them properly.