Hi pleistocene,
I would personally tend to avoid FBM (Foambymail) or any of their other sites. You can read more of my thoughts about them in post #2 here along with post #2 here.
Post #1 here and the information it links to has a list of steps that can greatly increase your odds of finding and buying a mattress that is suitable for what you need with the best possible quality and value in any budget range.
A mattress is only as good as the quality of the materials and components that are used in its construction so I would focus much more on materials used in a mattress than on brands. When you know the details of every layer of your mattress and are dealing with a reliable manufacturer or supplier … then you can make much more meaningful comparisons between mattresses regardless of the name on the label. As you can see from the information in the previous link … I would tend towards smaller local or independent manufacturers that are eitheq sold factory direct or through better sleep shops.
This would depend entirely on your body type, sleeping positions, and personal preferences. Thinner mattresses tend to feel firmer than thicker mattresses because there is less range for the foam to compress so it gets firmer faster but what may be too thin (or too firm) for one can work very well for another. The only way to really know for certain is to test it in person fully relaxed in all your sleeping positions to see how well it works for your personal needs and preferences. For most people a 4" polyfoam base layer would probably be too thin for comfort but it would depend on the type and firmness of the foams used and body weights and sleeping positions. If someone was lighter or say a back sleeper that really preferred a firmer mattress it could work well.
The choice between an innerspring and a polyfoam (or other) support core is also a matter of personal preferences. Both can provide good support in the right type and firmness levels but an innerspring is more … “springy” … and has a different response curve than than a polyfoam core which some people prefer. They are different but one is not “better” than another. The goal with all mattresses regardless of materials is that it relieves pressure, keeps you in your natural alignment, and matches your preferences in different materials, components, layering, and “feel”.
The support under the mattress would be important and a bunkie board (a 2" thick solid foundation) may help prevent the mattress from sagging depending if the surface the mattress was resting on wasn’t suitable already (solid surface, closely spaced slats etc). The type of foams and components used in the mattress would also make a difference in terms of the layers in the mattress shifting or sagging internally and stiffer and firmer foams would sag less (polyfoam for example would be stiffer than latex or memory foam). An innerspring with helicals (not a pocket coil) would also sag less and have stiffer internal structure than a foam base layer. Layers that were glued together would help prevent shifting inside the mattress. As you mentioned … it would probably also help to have a good quality cover that fit tightly around the foam layers rather than one that was a looser fit and a glued cover may also help as well.
If you let me know the city you live I’d be happy to let you know of some of the better manufacturers or retailers where you could do some testing in person.
If you are committed to buying online … then post #21 here has a list of some of the members here who specialize in online sales and are very knowledgeable and helpful in providing good guidance to their customers. They are also knowledgeable and transparent about the materials they use in their mattresses.
If you are looking specifically for an online memory foam mattress … then post #12 here has a list of some better options.
Who you buy from and their knowledge, experience, and ability to help you fit your circumstances and needs to a mattress can be just as important as which mattress you buy.
The first step IMO though would be to choose the types of materials you prefer from personal testing (in both the comfort layers and support layers) and go from there. Because of your weight restrictions … you probably wouldn’t want latex in the support core (a 6" core of latex in King Size would be close to or over your weight limit) but other than that and within your thickness restrictions (and 10" has plenty of flexibility in terms of design) you could probably use any type of material or combination that was used for mattresses on a regular bed.
Phoenix