Hi FranklinC,
Hopefully you’ve read this but just in case the first place I would start is post #1 here which has the basic informtion, steps, and guidelines you will need to make the best possible choices.
Innersprings are just a support component and in most cases the weak link of a mattress is in the upper layers which are above the innerspring. Good innersprings in a suitable design will rarely fail and in almost all cases it’s the use of low quality materials in the comfort layers that are the biggest durability issue. Even pillowtops can be a durable construction if it uses good quality and durable foam (which is often not the case in the major brands). You can read more about innersprings in this article and in post #10 here and you can read a comparison between them and a latex foam core in post #2 here.
If you are talking about an all latex mattress in king size with about 8" to 9" of latex then this would certainly be lower than you would normally find. To get to your budget or lower you would either need to consider a mattress with less latex or a latex hybrid that only uses latex in the top layers. There is a link in the “read first” to a list of members of the site that include many that sell latex mattresses or latex hybrids that are among the best quality/value in the country and can can also act as a “value reference” for a local purchase as well.
The better options that I’m aware of in the Atlanta area are listed in post #2 here but I don’t keep information or lists about pricing of specific types of mattresses (which changes too often for anyone to keep up with) so you would need to call these to see if any of these carry or sell a latex or latex hybrid mattress that is in your budget range.
This can be a very good option and has the advantage of being able to replace just a topper without replacing the whole mattress but I would make sure you have the chance to test the mattress/topper combination in person. If this is not possible then you are dealing with two variables (instead of just a mattress) and it can be difficult to predict how a certain topper will feel on a specific mattress unless you have tested similar combinations of materials in person. Without this … if I was considering this direction I would buy the mattress first and sleep on it for long enough to go through any adjustment period (for you) and break in period (for the mattress) and then use the guidelines in post #2 here and the posts it links to choose a topper based on your actual experience on the mattress.
I would not put a foundation or an boxspring directly on the floor no because it would restrict ventilation and could become a target for mold, mildew, or other “undesirables” such as dust mites.
I would either use a frame or add legs to the innerspring to elevate it above the floor except for very temporary use.
Phoenix