If it is at all possible, I think it is worth getting the best padding and mattress for yourself, that you can manage and afford. It doesn't take all my pain away or cure my illness, but it makes a difference in my life, I know.
This is important, because I bought a new mattress, and it was worn out and giving me back pain after 1.75 years. But since I was staying in bed for 20+ hours per day, the mattress was more like ~5 years, which unfortunately is when mattresses begin to be replaced.
One thing I love about my 15 year old Sleep Number mattress is that I can adjust the firmness. When I lose weight I need it firmer (number 60). If I am heavier or in a lot of pain I need it softer (30). You can't do that with memory foam. It also has a 20 year life.
You can find many Sleep Number beds available Used on Ebay and the company sells spare parts if something is broken. They also have sales. I purchased my bed for $700 as a demo on sale.