Hi the4thdimension,
For the moment I would hold off on a Restava mattress purchase until their current specs have been clarified so you can make more meaningful comparisons because I believe they have changed from what is on their website (see post #20 and onwards in this topic).
The tutorial post includes a link to a list of many of the better online memory foam options I’m aware of (in the optional online step). Post #13 here has more about the most important parts of the “value” of a mattress purchase. Which of your final choices is the “best bang for your buck” would depend on how each of your finalists compare based on the parts of your personal value equation that are most important to you.
I would normally choose the foundation that a manufacturer recommends because it would normally be a “safe” choice for their mattresses but if you also wish to look at alternatives then the foundation post here has more information about and sources for foundations that are suitable for different types of mattresses including memory foam mattresses.
Phoenix