Hi cardiacmommy,
Welcome … and I’m glad you found us. I would agree that after 17 years that your mattress is probably long past it’s “due date” 
There is more information about choosing an adjustable bed in post #3 here and the main adjustable bed topic that it links to that can help you choose an adjustable bed based on price vs features comparisons and also includes some retailers that you can use as good sources of information about the features of the adjustable beds they carry and as pricing references as well (in post #6 in the main adjustable bed topic). I would keep in mind that online advertised prices are often controlled so make sure you call them rather than just looking at websites.
There is also more information in post #2 here and the more detailed posts and information it links to about safe, natural, organic, “chemical free”, and “green” mattresses and mattress materials that can help you sort through some of the marketing information and terminology that you will encounter in the industry and can help you differentiate between them and answer “how natural is natural enough for me” or “how safe is safe enough for me” so you can make more “fact based” choices on the types of materials you are most comfortable having in your mattress. These types of issues can be complex and are generally specific to each person and their individual sensitivities, circumstances, criteria, beliefs, and lifestyle choices.
There is also more about dust mite allergies (which is the most common allergy related to mattresses and bedding) in post #2 here.
There is more about organic latex in post #6 here and more about organic certifications in post #2 here and the posts it links to but from a performance and durability perspective there is little to no difference between 100% natural Dunlop latex that is certified organic and 100% natural Dunlop latex that isn’t outside of the certification itself and the additional costs that it adds to the material. Of course choosing certified organic materials can still be a personal preference or a lifestyle choice that can be important for some people and there are some “non performance” benefits that are part of the organic certification process as well (see post #3 here).
This would also be a personal preference choice rather than a “better/worse” choice and both Dunlop and Talalay come in a wide range of firmness choices from very soft to very firm. There is more about the pros and cons of Talalay vs Dunlop in post #7 here but the best way to know which type of latex you tend to prefer would be based on your own careful testing and personal experience.
There is more about 100% natural Talalay vs blended Talalay in post #2 here.
12" of latex will be fine with an adjustable bed. You aren’t in a weight range that would “need” 12" of latex and there are many people that wouldn’t feel any difference between a mattress that has 3 layers and a mattress that uses 4 layers with the same top 3 layers with an additional extra Xfirm layer on the bottom although some people will and may prefer the extra layer either based on how it feels to them or because of the additional layering options that it would make available. There is more about the effect of thickness and/or having an extra layer in post #14 here and more about having more layers vs less in post #2 here. Some people also prefer a three layer mattress with a topper instead of having the extra layer inside the cover. There is more about the effect of having a separate topper vs having the same layer inside the mattress cover in posts #3 and #4 here and in this topic.
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 options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked 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 to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Phoenix