Hi mpicasso,
Welcome to our mattress forum :)!
Once you start gathering the knowledge to make an informed purchase is quite rare to not enter a “paralysis by analysis” phase…maybe we should have some kind of 5 stages of grief for mattress purchasing 
It will start making sense once you match the theory with the field testing and decide on the “must-have” mattress features. There are many variables that are interconnected and it is difficult to discriminate and arrange them in the order of importance for your own needs and preferences which are very unique to you. The mattress buying personal value equation may help to decide between your options. Yes, the Prana is quite expensive.
Depends on the springs … You are correct that pocket coils are the most motion isolating type of springs. Traditional innerspring mattresses are notorious for motion transfer issues for “light” or more sensitive sleepers. Memory foam is the best at motion transfer but latex, pocket coils, microcoils, buckling column gel, and even polyfoam are generally good as well but it will depend to some degree on the specific design of the mattress, on your relative weight ranges, and your sleeping style (such as how close you sleep together). Pocket coils are generally good for motion transfer, unlike innersprings that are linked with helicals which are generally poor to fair depending on the specifics of the innerspring and the foam layers above it. Just as an example most people will tell you that a mattress that uses latex comfort layers would be relatively good at motion isolation but there are also some that will tell you otherwise which is a good example of why once again the most reliable way to know whether a mattress is “motion isolating enough” for you would be based on your own careful testing or your own personal experience with both of you on the mattress in the positions you normally sleep in. You can find more about latex versus innerspring post#28here. And there is more about mattresses motion transfer in post #18 here .
Additionally, while both innerspring and a firmer latex core can be used as a support layer … each has very “different” characteristics but the most important differences are the ones you can feel and that you personally prefer. Both of them come in softer or firmer versions and in many different designs so an innerspring could be firmer than a latex core or the other way around depending on the specifics of the components you are comparing.
Nice to be able to DIY slatted frame to your own specifications. You may wish to visit the the Best Mattress Foundation thread here for some good tips. I have not seen any data to suggest that a 6" bigger size bed will have a measurable difference in durability. Most consumers typically revert to the same general area of the bed.
Congratulations on your weight loss through exercise and diet! … 45 lbs to go… IMO, I do not see any great benefit to buying a cheap bed and holding until the sought weight level is reached especially as you do not seem to deal with any pains due to alignment issues. As you lose weight the alignment also will improve so if possible I’d hold on a bit and instead experiment with what you have … maybe taking off the current topper and see how your body reacts to a firmer surface. Additionally, weight loss, although sometimes happens fast, is still a gradual process.
I am sure your dog would appreciate a little cooler sleeping environment as well…There is more in about the many variables that can affect the sleeping temperature of mattress/sleeping systems in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range. In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material.
Regarding Orlando, Palm Beach FL vicinities, I definitely think visiting the Ghostbed / Naturesleep store. and Bedmasters (almost 3 hours drive) they also have an all Latex Talalay mattress. that’s worth testing. Here are a couple of other threads regarding South Florida:
South Florida Mattress Store
Quality mattress retailers - Palm Beach / Marin Co.
Let us know if there is any other information we can help you with.
Phoenix