Hi Joy,
The first place I would start your research is the tutorial post here which has all the basic information,steps, and guidelines you will need to make the best possible mattress choice.
There are too many unknowns, variables, and personal preferences to use “theory at a distance” as a way to choose a mattress and the most effective way to choose a mattress is based on careful and objective testing (see mattress firmness/comfort levels in post #2 here). Nobody else can feel what you feel on a mattress although a good retailer or manufacturer can help you assess if you are in good alignment on a mattress.
Most people that are more sensitive or more concerned with chemical sensitivities or ecofriendly materials would tend to lean towards more natural materials but each person can have a very different answer to the question of “how safe is safe enough for me?”. While the issues of natural or synthetic materials, and mattress “safety” is a very complex one and there are no “right” answers for all people or circumstances … post #2 here and the other posts and sources of information it links to can be a good starting point to help you decide which materials you are most comfortable with.
Memory foam tends to sleep warmer than other types of foam for some people but it sounds like this may not be an issue in your case. It may also be worth considering a wool quilting layer in a mattress because wool is a great temperature regulator in both directions. There is more about the factors that can affect sleeping temperature in post #2 here.
Enso is a Chinese manufacturer (they are one of the many Zinus brands) that uses some lower quality/density foam in some of their mattresses so I would make sure you know the density of all the foam layers in any mattress you are considering and that they meet the memory foam guidelines in post #10 here.
Some of the better options and possibilities I’m aware of in the same area are listed in post #4 here. I think highly of Fly By Night and their knowledge and transparency and they are one of the options on the list.
You can read a little more about so called “plant based” memory foam or polyfoam in post #2 here. They are still memory foam or polyfoam that have a small percentage of one of the petrochemicals in “regular” memory foam or polyfoam (the polyols) replaced with plant based derivatives.
There is more information about some of the Ikea mattresses in post #3 here. I would be very cautious about their memory foam mattresses because they are less than 4 lb density.
It would depend on the specifics of the futon you used as a base and also on the specifics of the memory foam in terms of thickness, type, and density because all the layers of a sleeping system affect all the other layers so each difference in either one can affect how suitable the combination would be for you in terms of PPP. It can be a challenge to guess at the right thickness and softness for the topper in a topper / and base mattress or futon combination unless you have the chance to test them together before you purchase them but if you guess correctly then it can work well. It’s more risky than buying a mattress that you can test specifically for PPP.
Each person may be very different from the next so there is really no way to answer this on an individual level (but there is more information about this in the previous link). In general and based on the sensitivity of most people … offgassing only lasts a few days to a few weeks but different people are more or less sensitive to this and I have seen some people with MCS that are very sensitive to some materials (particularly memory foam but in some cases polyfoam as well) that need to air out some foam materials for a year or so before they can use it although this would be very unusual. Others tend to avoid synthetic foams completely.
With a slatted platform bed you may not need a foundation at all as long as the slats provide suitable support for the mattress you purchase. There is more about foundations and which ones tend to work best with different types of mattresses and some sources for each type in the foundation thread here.
Phoenix