Hi runningonempty,
This can be a risky approach and can have some surprising outcomes. Trying to “match” another mattress can be tricky (see post #9 here) and depends on the accuracy of the information you have about the mattress you are trying to duplicate (including the cover) and your ability to find the exact same layers elsewhere (type of latex, layer thickness, ILD, and a very similar cover). There is more about a DIY approach and the risks and costs that can be involved in post #15 here. It can be fun and a great learning experience if a “spirit of adventure” and experimentation is more important to you than the cost savings that may not turn out to be what you are hoping for if you make any mistakes.
All the layers in a mattress will compress to some degree but the top layers will compress much more than the bottom layer. The middle layer is more of a transition layer that can “help” both the comfort/pressure relief and the alignment/support of the mattress. The thickness of individual layers or the mattress as a whole though can make a significant difference in the “feel” of a mattress and how well it adapts to each person’s body contours and in how good a match it is in terms of PPP. There is more in post #14 here about the effect of layer thickness and the thickness of a mattress.
Post #2 here has more about having more options vs less but it’s not likely that at your weight you would “need” them.
When you are dealing with an online manufacturer and you haven’t tested a specific mattress or design in person (or something very close to the same design in your local testing) then a knowledgeable and experienced manufacturer or retailer will know more about how well the different options they have will “match” different body types and sleeping styles “on average” than anyone else (including me) and I would make your choice based on more detailed conversations with them (see mattress firmness/comfort levels in post #2 here) and then factor in the possibility of a layer exchange as part of the process that you may or may not need based on your actual experience.
Phoenix