Hi mtapp and welcome to the Mattress Underground 
We’re currently sleeping on a very old, very firm mattress and are looking to upgrade. We’ve went to a few stores to lay on different mattresses and really liked Naturepedic, especially the EOS Pillowtop
Sounds like your old mattress isn’t working up to its old potential any longer. As you know, any mattrsss you both decide on will be unique to you, based on your Stats (height, BMI, sleeping position(s) and any underlying health issues) - well done providing those! - and your PPP (Posture and alignment, Pressure relief and Personal preferences).
The EOS pillowtop, with 6" of latex and an 8" coil support, is a solid mattress which would be quite durable if you matched the comfort level to your preferences. You can see in the Mattress Specifications You Need To Know and compared to the Mattress Durability Guidelines, latex is one of the most durable materials you can have in a mattress.
we can only fit a queen mattress at this time but would like to upgrade to a king in the future. For that reason, the price seems excessive. We liked their plush coil/soft latex/soft latex build at the store but would like just a bit firmer. After researching DIY options, I think that might be a better route at this time
I’m currently thinking:
1" HD36-HQ 35 ILD foam (foambymail)
6’ Ascension X 1032 Pocket Coils (DIYmattress or LMF)
3" Medium Dunlop Latex 28 ILD (LMF)
2" Soft Dunlop Latex 19 ILD (LMF)
A quilted cover like on LMF
My questions are:
For the base layer should we have 1" or 2", and should this be the 35 ILD HD36 foam or the 50ILD Lux foam?
A DIY build can be, ultimately , the most rewarding way to customize a sleep system
for you and your partner, if you are willing to do the ‘leg work’ that it can entail….once you have a good idea of a comfortable arrangement of layers, ‘fine tuning’ is often required to adjust, moving layers around and trying out various permutations. Talalay latex is ‘bouncier’ than Dunlop (think angel food cake compared to pound cake). The firmness of the layers are a matter of preference for the most part; with your slightly higher BMI, if you go with a plush comfort layer you may ‘bottom out’ on the firmer layer(s) below. The base foam is there to support the structure of the mattress, and whether 36 or 50 ild is used, this should not affect the feel or support of the mattress. With both soft & medium ild latex layers you will be able to ‘swap’ them to experiment with how they feel for you.
The coil options are 8" L&P Quantum Edge Elite Bulsa, 6" L&P Caliber Edge, or the 6" BB Ascension X, what would the best choice be? Also there’s a large price difference between the Ascension coils on the two websites, are they the same product?
These are all different innerspring coil units. Typically coils are considered by their coil count, and gauge…lower gauge coils are more supportive for high BMI, and the higher coil count, the more responsiveness to multiple pressure points. With just the information you provided, any of the listed coil units would provide good support for consumers with your Stats, but, as they say, ‘your mileage may vary’.
I’m thinking all Dunlop for the latex because that’s what Naturepedic uses, and we haven’t laid on Talalay. Does the overall firmness of this seem correct (at least close)?
Is a quilted cover the wrong choice for a hybrid mattress?
Again, as Talalay vs Dunlop is really a matter of preference, as would be the quilted cover. If possible you may want to visit an in person showroom that carries different latex hybrids just to possibly get a feel for each type to expedite your consideration for your DIY build. Remember no one - here, in a showroom, anywhere- can tell you what type or firmness/comfort level of mattress is perfect for you, only the two of you can do this. Aside from the areas I commented on, the build looks good at this point, only your personal testing can tell whether it needs tweaking. Keep patient, and make notes of how each combination you try feels; it can be a little frustrating, but having a ew durable mattress customized to your exact preferences can pay off on the long run!
~ Basilio