Hi RobK and welcome to the Mattress Underground 
It certainly sounds like you have had a painful time on your old mattress! As we age our bodies can drastically change comfort and support needs, as you have seen with your old Serta; interesting the memory foam topper seems ‘as good as new’, as memory foam produced by the major brands can be of less dense inexpensive foam, and as these major brands are not very forthcoming about the materials and densities of their components, it’s difficult to assess their comfort and support for any consumer.
Keep in mind any mattress you choose will be unique to you, based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions), and your PPP (Posture & alignment, Pressure relief and Personal preferences). As you can see by comparing the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, latex is one of the most durable of materials, and a DIY latex mattress can be ultimately most rewarding, if you are willing to possibly need a little trial and error to zero in on the correct comfort and support.
I’m looking at doing a simple build using 100% natural organic latex toppers. They will be easy to swap and move around for cleaning. Three layers at 3" each seems to work well for a lot of people of my height and weight. I’m mostly stuck on deciding what densities to get. Everything I’ve read says to go with a medium soft mattress for my size, sleeping position and condition.
The place I plan to buy from recommended their soft, medium, firm combo which I believe is Dunlop. I had been thinking soft, medium, medium combo since I seen that worked well for someone else that was slightly bigger at 6’1" 200lbs
First, keep in mind that Talalay has a quite different feel than Dunlop; Talalay is considered ‘bouncier’ than Dunlop (the comparison is usually angel food cake compared to pound cake), so if possible you might try each type in a local showroom just to see which you prefer…and you probably want an organic, rather than natural, latex due to your sensitive sense of smell. Also, as everyone has unique Stats and PPP, just because a certain build worked for one person, even if similar BMI, does not mean it will work for you. Only you can judge what feels comfortable.
Here’s the plan so far:
Plywood platform bed
mattress rug or pad for airflow under the mattress
base layer of medium(26ild 4.7lb) or firm(34ild 5.3lb)
mid layer of medium(26ild 4.7lb)
top layer of soft(20ild 4lb)
zippered mattress cover, possibly padded, sometime in the future after testing mattress
There are no inherent problems with your build, but again, the densities of the latex are for you to decide…with a slightly higher BMI, a medium firm or firm support layer would provide a good support layer. The mattress pad is fine, and again the ‘firm-medium-soft’ or ‘medium-medium-soft’ are both workable solutions, it just depends on your preferences. The reason retailers ‘lump’ consumers into sleeping positions is because that, as part of a consumers’ Stats, are a good way to assess how much firmness they need. As a side sleeper, you need adequate support for your hips and shoulders; when you lay on a supportive mattress, your profile from the back should be the same as if you were standing up. Sinking too far in can cause alignment issues and muscle pains. In the same way, latex densities are also mostly a personal touch - as long as you have adequate support, the rest is fine tuning. A mattress you consider too soft could be too firm for another sleeper…this is why it’s important to take your time and also give yourself time with the new mattress to break it in and also accustom yourself to it. It can seem frustrating, but the end results are worth it. You might also want to read @Phoenix on Putting The Layers Together to get a better idea of how the layers all work.
I know you said you are looking at a local retailer, but several of the Trusted Members of the site, such as DIY Natural Bedding, CozyPure, and DIY Mattress among others, are highly experienced in assisting with DIY builds…if you provide a little info like your Stats, PPP, and mattress history, they can guide you through the process of finding the right comfort and support layers, and have generous exchange policies, as it’s not uncommon to exchange a layer if it doesn’t feel right for you.
One final note - check with whoever you decide to purchase your layers from to make sure your platform base is compatible, so that you don’t accidentally void any exchange of return policies if you need to take advantage of them.
You should hold off buying the mattress until you have the desired comfort/support balance …depending on the results of your experiments he may need to add/remove exchange some layering. It may be a good idea to look for a retailer with a good return/exchange policy.
I wish you success with your project and look forward to hearing your experiences once you have had some time with your build!
~ Basilio