D.B.
I think mattresses are like running shoes - it's not about a particular brand or style, it's about what fits. I think a good mattress is really important, but what's good for you might not be good for me. If you have such major back problems, I think it's unrealistic to expect a mattress to help much. I know you've posted about other health problems in the past (like allergies and hormones) so I think you might address the degenerative discs and arthritis in more effective ways. But I think they recommend that a mattress be replaced every 8-10 years anyway. I'd visit more than one furniture store (not necessarily one that only sells mattresses) and I'd check with your state's attorney general's office to see if there have been problems. I know that, in my state, there were many complaints about some hard sell/poor support mattress stores. You want someone who thinks they might see you again for other furniture needs - a mattress store knows they won't see you again for 10 years so it's all about this sale, today. There CAN be good ones that rely on referrals and hope you will come back for other beds in other rooms, but just check their reputations out beforehand. You also want to consider the needs and preferences of a person you sleep with - you may have different needs but there are mattresses that have different firmness settings for each side.
And you can ask these kinds of questions on this site without needing to connect them directly to parenting. This is a parenting site primarily, but members ask this sort of question all the time and it's fine!