If you get hot in bed, I'd definitely avoid any kind of memory foam/full foam as they retain the heat and in hot weather you end up feeling that you're sleeping in an oven.
The best mattresses are usually pocket sprung, and the more spring pockets they have, the better and firmer the mattress will usually be. Some people like a nice thick layer over their springs. Others with back problems sometimes like a nice firm mattress without too much padding. I think one of the best place to look for quality beds, if you have one near you, is John Lewis who normally stock a wide variety of quality mattresses, including their Own Label ones. . Be prepared to lose your self consciousness and lie on a mattress for a while, tossing and turning without any outer clothes/jackets on to feel whether it's firm enough and has enough padding on for you to be able to lie on it comfortably.