Hi saltlife,
There is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here but in general latex is the most durable of all the foam materials (memory foam, polyfoam, and latex foam).
There is more about the different types and blends of latex in post #6 here and more about 100% natural Talalay vs blended Talalay in post #2 here. If anything … blended Talalay made by Latex International can be a more durable material than 100% natural Talalay (in lower ILD’s especially) but the choice between different types or latex is more of a preference choice than a “better/worse” choice.
PLB has MAP pricing (minimum advertised pricing) but there are some retailers listed in post #32 here that sell the PLB mattresses online that you can call and talk with about their pricing for the same mattress to use as a “value” reference.
I would also be very cautious about buying the PLB foundation because they are not the best choice for a latex mattress (see the pictures here and here)
It would add additional softness and pressure relief yes but thicker and softer comfort layers also add to the risk that you can have alignment issues on a mattress over the course of the night which can lead to lower back pain when you wake up in the morning so I would make sure that you do some very careful testing on any mattress topper combination that has thicker and softer comfort layers to make sure that it’s not too soft for you.
Phoenix