Hi Northcountryfire,
This is a very controversial subject and there are certainly some people that believe this and while I have seen many of these types of comments about innersprings being a “safety” issue because of electromagnetic fields … I have never seen any specific evidence that this is the case other than some very non specific anecdotal stories that don’t provide any clear evidence that would directly relate to innersprings or metal frames. I would personally be more cautious with “active” electromagnetic fields (such as living close to high tension wires, holding a cell phone too close to your head, or sleeping near active electrical fields or wiring as examples) … but I personally wouldn’t have any issues with sleeping on an innerspring in terms of safety (although there would certainly be others that have a different opinion).
I would tend to avoid buying a mattress or futon with the intention of buying an additional topper where you can’t test the combination in person to make sure that it’s a good match for you in terms of PPP unless you have no other options available because of the additional uncertainty and risk involved (see post #2 here) compared to buying a mattress that doesn’t need a topper that you can test in person.
If you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP as a reference point (the same type and blend of latex in the same thickness and firmness levels and a very similar cover which can also make a significant difference to the feel and performance of a mattress) or use a “bottom up” approach (see post #2 here).
Phoenix