Hi Martina,
Post #8 here has some topper guidelines that can help you make the best possible choice for a top layer in term of thickness and softness using your experience on the base mattress or layer along with your body type and sleeping positions and also includes a link to a list of some of the better online sources for all the different types of latex I’m aware of.
You can read more about the different types of latex in post #6 here and about blended vs 100% natural Talalay in post #2 here.
There is more information about dust mites and allergies in post #2 here.
Phoenix