Frederixx,
The better options or possibilities I’m aware of in the Montreal area are listed in post #276 here. You are fortunate because there are many factory direct manufacturers that are relatively close by.
There is more about the different ways that one mattress can “match” another one in post #9 here that you can use as a frame of reference when you are comparing them. It’s relatively simple to compare the quality of the materials inside two mattresses as long as you know what is inside them (see this article) but of course comparing them in terms of PPP and how they feel is much more subjective and relative to each person’s individual perceptions so when you can’t test a mattress in person then the most reliable approach for this is to deal with knowledgeable and experienced retailers or manufacturers that will provide you with good guidance when you talk with them on the phone and to make sure you include the return or exchange policy as part of the “value” of your purchase so you are comfortable with the options you have after a purchase if it doesn’t turn out as well or “match” as closely as you hoped for. There is more about the different ways to choose a mattress (local or online) and how to reduce the risk involved in each of them in post #2 here.
This would depend entirely on the rest of the layers and how well the mattress “as a whole” matched your specific needs and preferences in terms of PPP. The thickness of the top layers of a mattress has little relevance by itself to whether a mattress will be a good match for any particular person because all the layers will affect the feel and performance of all the other layers in a mattress.
Firmness and softness is very subjective and there is no specific or common definition for “medium soft” between different manufacturers or different people so this really means that you are looking for a mattress that “feels” medium soft for you when you sleep on it regardless of how firm or soft it may feel to someone else with a different body type, sleeping positions, or individual sensitivity.
There is also no such thing as the “best deal” in general … only a mattress purchase that is the “best value” for you (regardless of whether it would be the best value for anyone else) based on all the criteria that are most important to you. There is more about the most important parts of the “value” of a mattress purchase in post #13 here.
The tutorial post also includes a link to post #21 here that includes of the better online retailers or manufacturers that I’m aware of that ship across Canada. Your best options will most likely either be locally in the Montreal list I linked or in the online list.
Phoenix