Mattress for mature couple

Hi, my wife and I are looking for a new king size bed and mattress. We are side sleepers, 5'5", 60 kg and 5'9" 75 kg respectively and both suffer from aching backs. We currently have an Ikea mattress (Which Best Buy a few years ago) on a hard divan that we now find is too hard. Which mattress would you advise and on which type of bed, a hard divan or or a soft one. Many thanks, David