Hi vjane89,
I’m guessing from your questions that you may not have read the tutorial post which is the first place I would start and has all the basic information, steps, and guidelines that you will need to make the best possible choices … and know how to avoid the worst ones.
You can read my thoughts about mattress reviews and why I wouldn’t use them to choose a mattress in post #13 here.
You can read my thoughts about buying from Costco or other big box stores in post #4 here.
There would be no way for you to know how well the mattress will work for you or how suitable it will be in terms of PPP (or your back issues) until you actually sleep on it so at least the return policy at Costco is a good one.
Outside of how well a mattress matches your needs and preferences though … a mattress is only as good as its construction and the quality of the materials inside it. Costco doesn’t have any information in the listing about the quality/density of all the layers in the mattress but you can read more about the Sleep Science Emma in post #2 here. As you can see the quality of the materials are low and certainly lower than I would consider.
Outside of the quality of the materials … Sleep Science also doesn’t meet the criteria I would use for a memory foam mattress (see post #10 here) because it’s made in China and isn’t CertiPur certified. You can read a little more about my phone experience with Sleep Science in post #2 here.
There is more about foundations and which ones are suitable for use with different mattresses and some good sources in the foundation post here. I would personally avoid using plywood or any solid non breathable surface under a mattress because of the lack of ventilation unless there was a compelling reason to do so that was worth the additional risk (see post #10 here). Memory foam mattresses will usually do best on a solid non flexing slat foundation or one of the wire grid types of foundations mentioned in the foundation post (and I would avoid a box spring with wire springs inside it that flex).
Phoenix