Hi freitazm.
Welcome to our
Mattress Forum!

Congratulations on your mattress order!
As you’re aware, you certainly are considering a great quality/value choice from Arizona Premium Mattress,
our Trusted Members here
which means that I think very highly of them and that I believe that they compete well with best in the industry in terms of their quality, value, service, knowledge, and transparency.
I am glad that you discussed with Arizona Premium the differences between the different options available in terms that are more specific to the layers to use as they relate to your needs. Your final product choice ~ 12.5”Ultimate Hybrid Pocket Coil with Dunlop latex both uses quality and durable componentry and one of the main benefits of this thicker mattress is that it can be more adaptable for higher BMI and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more "range" of compression without the mattress becoming too firm for heavier weights (or parts of the body).
The lumbar support area of the pocket coil will also help with keeping you in alignment as your higher BMI combined with a prone sleeping position would call for a more supportive lumbar section especially because the weight of hip/stomach area for a front sleeping position if not properly supported would throw your spine out of alignment in hyperextension of the lumbar area (The stomach is the weakest sleeping posture).
For the time being I ordered the Medium - I've heard that Dunlop is a little bit more firm than Talalay is. I figure if I can decide by the weekend I can always call and have the topper firmness changed if Ken hasn't already ordered from the supplier. I asked if medium-firm was possible in my order notes, to sort of compromise between the two, but I'm not sure if that's possible.
These are all good questions to ask and you certainly are making the right call using Ken’s expert advice which is based on his many years of experience in the industry. I would certainly make sure to run by him all the final tuning options before your mattress is shipped.
You are correct that Dunlop has a more supportive quality and it will be firmer and denser than Talalay. The biggest difference between them is what is called compression modulus. This is a measure of how quickly a foam gets firmer as it is compressed more.
The Firm Dunlop may feel much firmer than the medium Dunlop, but it is good to keep in mind that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components would be the best "match" for you in terms of "comfort", firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will "feel" to you, and that no "theory at a distance" can possibly be more reliable than your own careful testing or your own personal sleeping experience.
You would have 60 days to make an exchange in case this would be too firm or too soft and if an exchange is involved you may also need to take into account the UPS shipping costs.
You may also want to ask Arizona Premium about the type of foundation/platform that would best work with this.
I’ll be interested to hear about your final decision and any updates you might have after you receive your system and have a chance to sleep upon it for a while.
Phoenix