Hi smejenko1.
I have no idea what thickness (or firmness) may be best for any particular dog but I doubt you would need more than around 3 or 4" of latex that can be used by itself or with an added thin layer of memory foam. The latex rubber would be a good quality material and you can further differentiate the foams in terms of durability and feel in the same way as you would when choosing mattresses for humans. I would also probably use a good waterproof cover of some type. Post #6 here has more information about the different types of latex but all of them are durable materials compared to other types of foam.
Savvy rest has a 3" formed latex and a 6" shredded latex dog beds that could give you a few ideas of what most dogs like.
If any of the forum members have any experience with latex/memory foam dog beds it would be great to hear some feedback.
Phoenix