Hi GameMasterXO,
There is a list of the better choices in the DC corridor that I’m aware of in post #2 here.
If you are co-sleeping … it becomes especially important to choose materials that are as healthy and “safe” as possible (in terms of possible toxins or offgassing as well as performance) not only for adults but for babies and infants as well who have less developed immune systems and who need a firmer sleep surface. There is lots of information and links in this thread about mattresses and children which should help (even though they are mostly about mattresses for children and not as much for co-sleeping).
In general I would avoid memory foam completely and I would focus on more natural materials such as latex, innersprings, and natural fibers (in the quilting and ticking).
The iseries applause is an innerspring mattress which also contains synthetic low density polyfoam which will soften much more quickly than higher quality materials and it also contains memory foam which I would avoid. I would never suggest a mattress from a major brand which doesn’t disclose their materials and which are generally poor quality/value components. The 1.5 “comfort foam” and the "Hi IFD “support foam” are both lower quality/density and would be subject to early softening.
There are many Stearns & Foster “luxury firm” models in the Estate series (as you can see here) but I would also avoid all of them because they all use layers of polyfoam and/or memory foam in the upper layers which again would be the weak link of these mattresses because the polyfoam will soften and degrade faster than higher quality materials.
I personally wouldn’t even take the time to test mattresses where you don’t know (or can’t find out) the type and quality of the layers in it because even if you like the “feel” of one of these they won’t tell you much about your needs and preferences when it comes to other types of materials.
To recap though … I would choose materials that are safer which means innersprings, latex, and natural fibers. If your mattress does use polyurethane form … I would make sure that it was CertiPur certified.
I would choose a mattress that is a little firmer in the comfort layers than you would otherwise choose if you were sleeping without the baby. You can always add a topper (either foam or wool which is great for ventilation, humidity control, and temperature regulation) if it is a little bit too firm but children generally do better with a firmer mattress than adults. This doesn’t mean something that is “rock hard” but I would “err” slightly on the side of firmness rather than softness.
I would avoid mattresses that use a fire barrier where the manufacturer can’t tell you what is in it or what type it is. A wool fire barrier or a viscose/silica fire barrier would be the two “safest” choices IMO.
I would avoid the use of glues in your mattress unless you can confirm that the glue that is used is a water based non toxic and non offgassing glue.
Personal testing would be important to make sure that a mattress you are considering matches your needs and preferences for PPP (pressure relief, Posture and alignment, and Personal preferences) even though “just enough” in terms of pressure relief is the goal. Which type of mattress would be best for you will depend on your body types, sleeping positions, and preferences and because there are so many variables between people … there isn’t a “formula” that can “input” your specs and come up with a “perfect” layering. Most manufacturers and better sleep shops though are good at helping you make good choices and will help educate you about the benefits of the different choices they carry. I would make sure you call them first to talk about your criteria and get a sense of the general level of knowledge and service you can expect if you decide to visit them.
As a backup … there are also some good online options available to you if you find that your local options are not quite what you want in terms of either value or materials. They are listed in post #21 here. One of these (Cozy Pure) has a bricks and mortar store about 4 hours away from you but they make some very high quality premium mattresses that would certainly fit the bill (and are customizable) if they are inside your budget.
Hope this helps and if you have more questions along the way feel free to post them 
Phoenix