Hi Pspa123,
The better options or possibilities I’m aware of in and around the Boston area (subject to the guidelines here) are listed in post #2 here. You certainly have some good local options available to you.
Not necessarily no. The weakest link of most mattresses is in the upper layers of the mattress which are most subject to softening and breakdown and not in the support materials or components in the mattress and the biggest durability issue with most innerspring mattresses is not in the innerspring itself which is most likely still in relatively good condition. A good quality innerspring in the right design can be “built like a tank” and if the comfort layers are high quality materials and it is a suitable design (especially a two sided mattress) it can last for many years even for those that are in very high weight ranges (400 lbs or higher)
The choice between a good quality polyfoam support core and an innerspring support core would be a preference issue (assuming that both of them are high quality materials or components) rather than a “better worse” issue. There is more about the different types of innersprings in this article and in post #10 here.
Yes … HD polyfoam is generally fine as a support component in a mattress and for higher weight ranges or for applications where greater durability is important then a minimum of 2 lb density would generally be a good idea.
There are some foam quality guidelines in post #4 here that may be helpful and post #4 here and the posts it links to also has much more information about the many variables that can affect the durability and useful life of a mattress relative to different people.
Phoenix