Hi gingerperty,
If you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed are a good match for you in terms of PPP as a reference point or use a “bottom up” approach (see post #2 here).
Wool can absorb up to about 30% of its weight in moisture and still feel dry against the skin and help maintain a lower humidity microclimate on the sleeping surface so it’s a very good material in humid environments. Wool is also a very good temperature regulator in both directions and has been used historically in very cold environments along with hot desert environments (in thinner layers) to regulate temperature.
There is more about the pros and cons of quilted covers vs more stretchy knit covers in post #12 here and the posts it links to. The choice between them would be a personal preference.
There is also more about the pros and cons of different types of fibers and fabrics in post #7 here and the posts it links to as well.
Phoenix