Hi crschwartz,
I can’t give you a specific suggestion without more detailed reference points and information about you (height/weight, body types, sleeping positions and descriptions of the specific “symptoms” you are experiencing and where they are) but there are some guidelines about choosing a topper in post #8 here.
The choice of material would be a matter of preference and I would focus on materials that you have tested and like rather than materials you are not familiar with. Once you know the material you prefer … I would use the highest quality version of that material that I could comfortably afford (or at least be aware that lower quality versions will be much less durable). If you do go in a direction of an untried or unfamiliar material … then a merchant that offers a refund may be important unless you are fairly certain that you will like how it feels and performs.
With latex … I would tend to avoid something as soft as 14 - 15 ILD because of durability issues and because of the risk of needing to use a topper that was so thick and soft (to isolate you from the firmness of the lower support layers) that you would be risking alignment unless you had specifically tested a particular mattress/topper that used it. For most people I would be looking at 19 - 24 ILD in the “plush - soft” range and 28 to 32 ILD in the medium - medium firm range(of course what is soft and firm for each person will depend on their weight, body type, perceptions, thickness, and sleeping positions).
A good “general” or “average” suggestion (in the absence of information that would indicate otherwise) would be 2" in the firmness level that seemed closest to your own preferences because is it enough to make a difference but not so thick that it has a bigger risk of creating alignment issues but bear in mind that an “average” may not apply to any specific people or set of circumstances.
Some of the better sources for toppers I know of are in post #4 here.
In the DC area it may be worth contacting http://americanfoamcenter.com/ to see what they have available.
If your topper is latex … then a cover is essential yes to protect it from oxidation and prematurely degrading. I would also use a cover on other types of foam to protect them from damage and premature wear as well.
Many foundations are made just slightly smaller than the mattress so that they fit into a frame better but there is some variation here. I would measure your bedframe exactly and then talk with the supplier you are considering to make sure that the exact measurements will fit into your bedframe. If you do go with a KD foundation and because your mattress is “all latex” … I would make sure that the gaps between the slats were no more than 3". The foundation thread here may be helpful.
Phoenix