Hi Sergio G,
Welcome to the Mattress Forum! all the way from Germany 
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices. While many of the specifics mentioned on the forum in terms of manufacturers and sources would be more specific to North America … the basic guidelines would be common to any country (although you may need to translate some of the specs into metric such as lbs/ft3 to the Europoean equivalent of kg/m3 (there’s a density conversion page here)
[quote]Reading of Memory Foam I have been fascinated by the idea of “weightless” because it gives me the idea of a very relaxed sleep like in a, hmm, how do you call it in English, sensory deprivation chamber?
But I am also reading about the possible problems.[/quote]
The type of mattress or materials that are in your mattress are really a matter of preference so some people may have a strong preference for the “feel” of memory foam while others would never consider it and don’t like it at all. Your own testing and experience is really the best way to decide which type of mattress you prefer.
Regardless of which type of mattress or materials you prefer, it’s always a good idea to make sure that any mattress you are considering is a good match for you in terms of PPP and also uses good quality materials … especially in the upper layers which are the most likely to soften or break down prematurely. All materials have lower quality versions and higher quality more durable versions and you can’t “feel” the quality of the materials when you test a mattress because higher quality and lower quality materials can feel the same … at least for a little while. High quality materials can last for longer than the 5 years you are looking for while low quality material may last considerably less. Some of the links in the tutorial post include the minimum quality/durability guidelines I would suggest for different materials.
[quote]I was thinking about a Mattress with Polyurethane) below and Memory above.
Do you think I am right?
But, which thickness, density, shape and firmness should each of the two layers have, to give me what I need?[/quote]
Again … the type of materials you prefer are not a matter of right or wrong but a matter of personal preference. There are also too many unknowns, variables, and preferences between different people to use any kind of formula or “theory at a distance” to decide on which mattress or what specific design would be best for any individual person. There is more about this in mattress firmness/comfort levels in post #2 here.
If you follow the steps one by one you will have the best odds of buying a mattress that is the best match for the criteria of your personal value equation that are most important to you.
Phoenix