Hi Marz,
There isn’t a specific thickness that anyone requires because it would depend on many factors including the type and firmness of the layers and on the body type, sleeping style, and preferences of the person. Overall … there are too many variables involved and I would avoid trying to figure out which mattress may be a good match for you based on specs (either yours or a mattress) or theory and use your own careful and objective testing instead (see mattress firmness/comfort levels in post #2 here). In very general terms … heavier body types may do better with thicker mattresses but this is only very generic because it always depends on the specifics of the mattress and the person (see post #14 here for more about the effects of thickness).
The better options and possibilities in the Okanogan valley that I’m aware of (subject to the guidelines here) are listed in post #4 here.
Phoenix