Hi laughingsquirrel, and welcome to The Mattress Underground 
Before I pull the trigger on purchasing my layers for my DIY config I’d thought I’d check first.
I am 5’10 and 145-160 (I tend to fluctuate in all honesty). I am a side sleeper. In the past I’ve been comfortable on a 6" linenspa (hard as a rock with Bonnell springs no doubt) with just a 3 or 4" memory foam topper. Strangely I replaced the topper at some point and then got more pressure issues, particularly my hips. I figured maybe my original topper was thicker or better quality.
A DIY project can be the most rewarding, as well as the most frustrating, type of mattress to choose, it sounds like you want to repurpose the components from your current Posturepedic. It can take a lot of trial and error to get the right feel but a bit of basic knowledge and a keen observation helps you go a long way. Remember any mattress is unique to the sleepers using it, Stats (height, BMI, sleeping position(s) and any underlying health conditions) and their PPP (Posture and alignment, Pressure relief, and Personal preferences). You might benefit from reading the Mattress Durability Guidelines, and the Mattress Specifications You Need To Know as well as this article on layers to get a better idea of how the layers all fit together.
I prefer coiled hybrids (since I liked the linenspa) so I got a Posturepedic Chantalle (from around 2016) which has “embrace coils”. They are in decent shape. They are 6" coils sitting on 1" or so foam, this mattress has foam edging. It’s listed as a medium bed (it had 1" memory foam and 1" egg foam in the topper). I am using the coils as the support base.
The LinenSpa and Purple mattresses are ‘mattress in a box’ products. They are not transparent in their materials nor their components…any excessive amounts of inexpensive or low-density memory foam is liable to wear prematurely, leaving ‘divots’ or ‘ruts’ which can cause aches, pains and even spinal misalignment. Additionally, the compressed bed-in-a-box type mattresses are stored compressed in unknown environments for unknown amounts of time, which can also cause wear and damage. For memory foam, I would make sure any foam in a potential mattress your DIY has no more than an inch or so of lower grades foams (for memory foam no lower than 4 lbs/cuft for normal range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have adequate support.
I plan to add (bottom to top) 2" medium D, 1" nanocoils, 2" soft D and a cotton cover.
Would that work? I like to lay on top more than sink in so that’s why I opted for soft D instead of soft T.
I am not sure what you are trying to achieve by sandwiching the nanocoils between the two Dunlop layers but there is no reason why that should not work, except that you may need to protect the foams above and below from being damaged with some sort of fabric or thin cushioning layer. You can read more about microcoils here. The foam edge around the 6" coils may also present some issues as usually the density for the foam edges might be quite low and break over time.
For your low BMI, there is nothing else that jumps out about your planned DIY layers, as long as they feel comfortable for you. Keep in mind Talalay Latex is considered ‘bouncier’ than Dunlop; you may need to switch some layers around and even exchange comfort layers as you ‘dial in’ the right comfort level. You also may want to consider purchasing your layers from one of the Trusted Members of the site like DIY Mattress(who have many years of experience assisting DIY consumers to select the right components, as does MFC if you are Canada based. We have a large number of DIY enthusiasts on the forum as well, who can perhaps also comment on your project.
~ Basilio