It depends on what you like. I purchased a custom made Japanese futon style mattress which is quite firm, and I really like it. It was a fraction of the cost of a typical mattress. Like, less than $1000 whereas really expensive mattresses can be almost $10k, maybe more.

I don’t have back problems, but I think the futon with it’s firmness helps with that. Neck problems could be caused by having too much or too little pillow.

This is the one I got, in case anyone wants to research: futonz.co.nz/latex-core-futon/