Don't know where to begin!

Hi Guys!
Really desperate for some guidance and was wondering whether you could assist me. It has become obvious that I need to replace my 20+ year mattress (embarrassing, I know!) but I don't know where to begin. The current mattress is spring orthopedic mattress which is not comfortable at all.
I have never purchased one before and to tell you a bit more about myself is that I am male, 80+kg and 5 ft 11 in. I was looking at the Sealy Geltex Avola as I wanted something that is supportive yet still soft that doesn't give me a sinking feeling. I want more info on whether I need pocket springs with the geltex cover. I do suffer from lower back pain/soreness most mornings so have been instructed by my osteopath to sleep on my side to alleviate morning stiffness and disorientation, which has been helping greatly.
I was wondering if you could help me make a decision as from reading some advise pages on your site I can see you are incredibly helpful in supporting customers making the right decision. I have never slept on memory foam much however can tell you it does make me hotter than i would like to be, so latex as it states is a lot cooler may be in of interest, but I look forward to your response.
Thank you!