Hi sleeplikeabear,
If you are considering a DIY mattress I would make sure that you’ve read option 3 in post #15 here to make sure you have realistic expectations of success and the trial and error that may be involved. The “best” approach with a DIY mattress is a “spirit of adventure” where the process itself, your experiences, and what you learn along the way are more important than any cost savings which may or may not happen depending on whether you purchase any layers or components that end up needing to be replaced.
Oeko-Tex is a certification that tests for harmful substances and VOC’s. All the latex you are likely to encounter (whether it’s Dunlop or Talalay and made from synthetic, natural, or blended latex) will all have an Oeko-Tex standard 100 certification or a similar certification from Eco-Institut. Both of these are a little more stringent than the CertiPur certification that is usually used for memory foam and polyfoam. You can see what Oeko-Tex tests for here. In other words … if it’s latex of any type … then it would be a “safe” material.
There is more about the different types and blends of latex in post #6 here. All latex is a durable material compared to other types of foam.
All of the firmness ratings would depend on how you perceive them which can vary widely with body type, sleeping positions, and individual sensitivities and preferences. Different people will have a very different experience on the same material and something that feels “too soft” for one person can feel “too firm” for the next. The firmness of a mattress will also depend on how each layer interacts with all the other layers of a mattress and I would also be aware that there are different “types” of firmness and softness (see post #15 here) and that ILD alone isn’t the only thing that determines how firm or soft a material will feel (see post #4 here).
The component post here includes the better sources for different types and blends of latex and covers that I’m aware of.
Phoenix