Hi gmgdwrnchblck3,
I think 3" would be a good choice in your circumstances. The choice between latex and memory foam (Tempurpedic is just a brand name like Kleenex and not a type of material like “tissue”) is really a matter of personal preference. You may want to test some mattresses locally that have both in the top layers (make sure there is no polyfoam over them so you are feeling the latex or memory foam itself) to see whether you prefer a fast response material like latex or a slower response material like memory foam.
Either way … in the case of latex I would be looking at a medium firmness rather than softer because of your heavier weights or in the case of memory foam I would tend towards 5 lbs density or higher (or if it is a good quality gel memory foam then 4 lbs would be fine).
Post #8 here has some topper guidelines (and again I would tend to agree with 3" in your case) and post #4 here has some of the better topper sources I’m aware of regardless of which material you choose.
Phoenix