Hi fixmenow,
The minimum quality/durability guidelines I suggested would be the lowest quality/density I would suggest regardless of your weight range (and I would look for higher than the minimums for those that are in higher weight ranges).
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They also don’t provide the density of the foam layers in the mattress. I would want to know the thickness and the density of all the layers in any mattress I was considering and I would avoid any mattress where a retailer or manufacturer either isn’t able or willing to provide it to you because it would be a very risky purchase. Would you buy a computer that just has “a hard drive”, a “CPU”, and “some memory” without knowing some more detailed specifics?
It would depend on the reason you needed a topper. You can also see my comments about choosing a firmer mattress first with the intention of adding a topper later in post #2 here.
In most cases I would avoid this approach because of the uncertainty involved with two purchase choices instead of only one and choosing a topper that would be suitable in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (unless you can test the combination in person or you are purchasing both online as a “set” that is designed to work together and they both have a good return/exchange policy) and then use the option to add a topper as a “backup” strategy in case your initial choice is too firm and doesn’t turn out as well as you hoped for rather than a “primary” strategy.
It can also be more costly than just buying a mattress which includes the same materials as the topper as a layer inside the mattress itself and you would be avoiding the risk of having additional materials in your base mattress that you may not need or that may be lower quality foam (that can soften or break down more quickly) or that may be softer than what would be ideal for a transition layer under a softer topper. I would also make sure that you can find out all the specifics of the materials inside the base mattress so you can make sure it meets the quality/durability guidelines here and that there are no lower quality materials or weak links in any of the materials or components in your sleeping system that could compromise the durability and useful life of the mattress.
Having said that … if you do decide to go in this direction and end up choosing a suitable mattress/topper combination which uses good quality and durable materials and turns out to be a good “match” for you in terms of PPP (and there are no lower quality materials or “weak links” in the mattress/topper combination) then it would have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is likely because a sleeping system will tend to soften or break down from the top layers down) or if your needs or preferences change over time and a topper can also help extend the useful life of a mattress underneath it as well.
Phoenix