Hi aaintx,
Higher weights tend to need firmer foams both in the comfort layers and the support layers because you will compress them more easily than someone that is lighter. I would also make sure that the materials in your mattress are high quality so that the upper layers especially don’t soften or break down too quickly. I would particularly make sure that you do some careful and objective testing for PPP so that you don’t buy a mattress that is too soft for you because a mattress that is too thick or soft in the comfort layers especially can be very risky for stomach sleeping. The goal is “just barely enough” softness and thickness in the comfort layers to relieve pressure points on your side so there is less risk of alignment issues when you sleep on your stomach. You can always soften a mattress that is too firm but it’s not really possible to make a mattress that is too soft any firmer.
Your budget as you know is quite low (and it will also depend if you need a foundation in your budget as well which would reduce the amount available for the mattress) so you will probably be looking at either an all polyfoam mattress or a traditional innerspring mattress with polyfoam comfort layers rather than memory foam or latex which would normally be in higher budget ranges.
One of the most important parts of your research will be the initial research you do on the phone before you go anywhere to test mattresses. The list of better options and possibilities in the Dallas / Fort Worth region I’m aware of are in post #4 here.
I would first call the ones that interest you or are withing the range you are willing to travel (or all of them) on the phone and let them know your criteria including price (like you have here). I would also ask them if there is a time that they are likely to be less busy so that you can do your testing with the least number of people around. I would then visit the ones that you believe would be the best options or that “connect” with you and your criteria the best and appear to have mattresses that you want to test and that they believe would be worthwhile options.
The more you can do on the phone before you visit any of them the more effective you will be when you go to test mattresses.
Phoenix