Hi jGary,
I would say that the general strategy you are looking at is good but if you are getting these materials combined for under $700 (depending on the size of the mattress you are building) then I would question whether you are buying “top of the line” materials. There is also quite a bit of slow response materials (5") in your suggested layering which can be quite risky
Therapedic has a mattress which is similar in construction to what you are suggesting (except it has an inch of latex under 2" of gel memory foam and 2" of 5 lb memory foam instead of 3" of the 5 lb memory foam with a 6" 1.8 lb density polyfoam core). You can see it here (at a “lower than normal” price).
Post #2 here includes some of the better options in and around the Baltimore area including American Foam Center which may have the materials you are looking for as well. A couple of the outlets also carry Therapedic.
Post #4 here has a list of some DIY sources that would also allow you to put the mattress together on your own if you are committed to the specific layering you are suggesting (assuming they have advantages over the sources you are considering).
The real goal and challenge is to make sure that the layering you are suggesting would be suitable for your specific needs and preferences without testing how they interact together with you and with each other and the mattress cover that you choose. Like all mattress materials … memory foam (and gel memory foam) is only a general category of foam and there are many foams with a wide variety of different properties and feels in every category.
Gel foam in particular is “marketing driven” at the moment and while it may have some advantages over some (not all) “non gel” memory foams and there are also better and worse formulations of gel foams as well … any material that is used in the top layers of a mattress, no matter how “cooling” the marketing says it may be … is only one of several factors that is important in regulating the ventilation and sleeping temperature of a mattress.
Phoenix