Hi jblythe,
The Flex MLE seems to me to be a good “budget” choice for a latex mattress with 5" of latex on top and then higher density polyfoam underneath and it doesn’t appear to have any polyfoam in the comfort layers (although I’d check to make sure). The value would depend on the price of course and how it compared to other mattresses like this but overall the materials seem like a good combination. If it gives you good pressure relief and alignment in all your sleeping positions then this would be a good compromise that didn’t have the expense of a full latex core but had the feel and benefits of a latex comfort layer.
Post #47 here and post #2 here should help with various foundation options.
If you meant one of these when you mentioned a “metal frame” … then it should be fine although they wouldn’t be my favorite choice.
Phoenix