I bought a latex mattress with a 4" topper of memory foam from
www.foamorder.com 7 years ago or so. I'm about 200#, and the memory foam
gradually developed a sag in the center, I made pillows out of the ends and
threw out the rest. But, it was nice while it lasted. Gave the latex
mattress to a friend in need, it's like new.
Dunno what density the memory foam was, it's whatever www.foamorder.com has,
but if the stuff you get is like theirs, I'd expect it to start to show wear
in a year, and need to be replaced in five.