Type of mattress for ottoman storage bed kingsize

Hi, looking to purchase kingize mattress to go onto sprung slatted base ottoman bed. Need something firm i think and cool find memory foam too hot... have been thinking about a pocket sprung mattress... i am 5ft weigh about 13st and husband is 6ft and a out 16.5 stone. We both suffer with back pain and overall aches and pains. Our little boy also sleeps with us in the middle. Any suggestions? And what kind of cost would webe looking at?
The mattress we have at monent is pocket sprung with memory foam which seems to be going out of shape- only had for a month so will be going back.