Hi Del,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choice … and know how and why to avoid the worst ones.
The name of the manufacturer of a mattress doesn’t really make much difference because outside of how suitable a mattress is for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) … a mattress is only as good as the quality of the materials and components inside it regardless of the name of the manufacturer. There is more about the most important parts of a successful mattress purchase in post #13 here.
I believe that the Macybed is made by Serta and while I would never buy a mattress for personal use without knowing the quality of the materials inside it … it would be safe to assume that the quality of the materials inside it are very low quality and certainly wouldn’t be suitable for regular use for someone of your weight. It may be suitable for a guest bedroom where it was used only occasionally but even here you could probably do much better than buying any mattress where you don’t know anything about what is inside it and where the odds are very high that the quality of the materials is very low.
Phoenix