Hi Nicholas,
Welcome to the Mattress Forum!
As you’re probably aware (but maybe not as you’re new to the site), I certainly can help with “how” to choose, but I can’t help with “what” to choose, as there are simply too many variables and personal preferences to take into account.
Regarding the Koala, looking back at the updated post of monopocalypse, I don’t believe the specifications there are correct. The base foam of the mattress is a polyfoam of 38 kg M3 (2.37 lb.), which would certainly be durable enough for someone with your BMI. However, I don’t know the thickness of this foam layer. I’m also unsure about the upper layer of the mattress. They don’t list the thickness or the type of foam, but in another part of their web site they state that this mattress uses two types of polyurethane foam. They seem to be describing some of the new breed of higher-density polyurethane foams that are soft but quite breathable, but I’m not sure, as technically memory foam is a polyurethane foam as well. You’d need to find out this information before I could comment upon the quality of the componentry in this mattress. I don’t have any contacts with Koala, as I focus primarily on products in North America.
If the specifications are correct for the Sleep & Co. S3 (which I believe they are), you can see my comments above that both the memory foam layer and the polyfoam base are lower in density than what I would normally recommend, and would only be considered if this were more of a budget-friendly mattress, knowing that you’d be trading durability for price point.
You may want to read the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
To help you compare, always make sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Phoenix