Hi motor58,
The specs you listed only show the firmness of the foam (40# is the IFD or firmness not the density). The density is the most important part of the durability of a foam and any density can be made in a wide range of firmness levels. I would probably call or visit Eastside to see if they can tell you more about the specifics of the mattress. In the price range you are looking at $1300 certainly wouldn’t be a “cheap” mattress at all if it was using polyfoam and didn’t use specialty foams like latex or memory foam. The specs you need to assess the quality/durability of a mattress are in this article and the foam quality/density guidelines that I would suggest using as a reference point are in post #4 here.
That really depends on the specifics of the mattress and it’s thickness and on the specifics of the flexible slat system but with thinner mattresses like yours it could make a noticeable difference in both the feel and performance of a mattress yes.
The foundation post here and the posts it links to in the second paragraph has more information about the types of foundations or support systems that generally work best for different types of mattresses but I would tend to use a slatted foundation that provides some airflow under a mattress unless there is a compelling reason to choose a solid surface which doesn’t allow any airflow (see post #10 here).
The better options and possibilities I’m aware of in the Seattle area are listed in post #2 here.
Phoenix