If you don't want to be too warm, and you want better support, then you should go for pocket sprung with memory foam. These are far more supportive than the foam only type. And as long as you get one with full size steel pocket springs, rather than reflex foam encapsulated pocket springs, it will also be far cooler.
If you can find one get a two sided mattress as it will last far longer if you can turn it.
If I were you, if you want a good quality mattress at a low price, avoid big brand names, and big retailers. Instead shop on line, and go for a smaller independent manufacturer. Just remember to buy from a retailer who gives a proper satisfaction guarantee, that allows you to unpack and sleep on the mattress.
For more advice have a look at this site. It is just an advice site, and isn't trying to sell you anything: www.mattressadvice.co.uk