Hi Expat,
Welcome to the Mattress Forum! 
Latex softness and density have an almost direct relationship. For example, a king 6" latex mattress (and you wanted something a bit thicker) of Dunlop at a 75 kg/M3 density (a “medium” comfort), would weigh approximately 100 pounds itself, and that is without a covering, wool or FR layer, or any other materials associated with the finished mattress. So an all-latex mattress is most likely out of the question for your bed base.
Your best bet would be a mattress using a polyfoam core with latex or a “latex-like” ultra high-density polyfoam on top. Even a model using an innerspring unit with a thin layer of latex on top might be exceeding your weight limitations.
At a quick glance through the members of the forum, the Nest Bedding Hybrid Latex is about 130 pounds in a king size. A king OSO mattress has 1.5" of latex in it and it 76 pounds in a king. You could choose a polyfoam core base from Sedona Sleep ADMIN NOTE:Removed 404 page link | Archived Footprint: sedonasleep.com/sedona-sleep-base-mattress/
at approximately 50 pounds in a king and then use a 3" 19 ILD Talalay topper at approximately 32 pounds. The Luma Sleep Hybrid Mattress and the My Green Mattress Natural Escape might be items to consider, but I don’t have the weights on these and they might be borderline for your application, so you’d want to phone them to be sure. If you do decide to purchase from one of the site members, be sure to request your Mattress Underground discount.
The Dual 9 Firm mattress uses memory foam and is not a latex-style product, so I’m not sure that it would be the style of mattress that you’d desire.
You can read a little more about Magniflex in post #6 here and post #2 here and post #3 here and a forum search on Magniflex (you can just click the link) will bring up more information and feedback about them as well.
If you are considering buying a Magniflex mattress I would make sure that you confirm the quality/density of all the layers (see this article) in the specific model you are considering so you can confirm that there are no lower quality materials or weak links in the model you are looking at and make more meaningful comparisons with any other mattresses you are considering based on suitability, durability, and all the other parts of your personal value equation that are most important to you.
Phoenix