Hi ann,
You should easily be able to make a choice that will last you for longer than 4 - 5 years … especially if you choose a firmer mattress with less “suspect or unknown” materials in it and then add a topper or if you choose a mattress that uses higher quality materials in the upper layers. A topper will also extend the life of the layers below it because it will absorb much of the wear and tear of sleeping on the mattress.
The other advantage of choosing a firm mattress with only thin comfort layers (so there is less lower quality or “suspect” materials) is that it will be significantly less than the softer versions so what you save can be used to add a high quality topper which can end up providing a better quality sleeping system than a much more costly mattress.
Post #2 here and the posts it links to has some guidelines for choosing a topper and some good sources as well. I would make the mattress purchase first and then use your experience on the mattress (which may be too firm) as a guideline for choosing the most suitable topper.
It would also be worth calling some of the manufacturers or retailers on the San Francisco / Oakland list in post #2 here as well although I don’t know which of them specifically carry toppers or which types so you would need to check websites and/or make some phone calls. There are some good quality/value mattresses available in the area if you decide to go in that direction.
Phoenix