Hello EJS.
Welcome to our forum. 
This will be quite difficult on an adjustable bed base, unless he sleeps “upside down” (his head at the foot section of the bed) with the leg section slightly elevated. It sounds odd, but is actually is comfortable for some prone sleepers.
Naturepedic is also a high quality option and I think highly of them as well although they are generally in a higher budget range compared to other similar mattresses. A forum search on Naturepedic (you can just click the link) will also bring up more comments, information, and feedback about them as well.
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them (like the Naturepedic you tried and liked), any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
There is more about the different ways that one mattress can “match” or “approximate” another one in post #9 here but the only way to know for certain that two mattress will feel the same is if they both contain the same types and blends of latex, all the layers are the same thickness and firmness, and they both have a very similar cover (including any quilting). Once there are differences between any of the specs between two mattresses then “approximating” a mattress based on specs can become much more difficult and the most reliable way to know how they would compare for any particular person would be based on your own side by side testing and personal experience because different people can have different opinions about how closely two mattresses with differences in their designs and components compare to each other because body type, sleeping positions, and individual sensitivities can also play a role in how they feel and compare. Of course, if you’re shopping online, comparing them side by side isn’t a possibility, so I’d again refer you back to phoning any potential manufacturer you’re considering.
I’m assuming that you’ve read the mattress shopping tutorial here but two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
When you’re considering items online and are unable to try out these products in person, be sure to investigate the return/exchange policies and be informed up front before you consider making a purchase.
Regarding the potential use of a topper, you certainly can always make a firm product a bit more plush in the future, but I would focus initially on mixing as few variables as possible and look at the mattress alone itself when buying online.
I spoke with Ken at Arizona Premium Mattress Company and he admitted that they always want to be up front and inform consumers that while they accept returns it is a bit more difficult to return a spring mattress. He mentioned that they have never had anyone return one of their spring mattresses so far, but as long as you have a good shop-vac you can return anything that they make. They have a popular video on youtube that describes the process, and it doesn’t look that difficult, especially if you have two people. Again, the key is that being an online company that they offer the ability to exchange/return products.
That would come down to a personal preference. Talalay, with a more buoyant feel, is popular as a top layer of a mattress. But if you prefer the feel of Dunlop, there is nothing wrong with using Dunlop as an upper layer.
I’m not sure what Luna bed you are referring to. Can you provide a link? But if you say you prefer Dunlop and they only use talalay, then the point for you personally would be moot.
As you know I think highly of Arizona Premium Mattress Company and Nest Bedding and they certainly make some great quality/value mattresses but as you probably know from your reading here the quality or value of a mattress has little to do with how well you will sleep on it because a mattress also needs to be a good match for you in terms of PPP as well.
Phoenix