Hi dhanley,
In general I would suggest a medium to medium firm range.
I would use the quality/durability guidelines in the previous post I linked.
Any mattress that uses good quality and durable materials that would still be in relatively good condition when they become teenagers but is too firm for their needs and preferences when they get older and begin to develop adult proportions and preferences would make a good base mattress for a softer topper.
This part would be a matter of knowing the specifics of the materials in the mattress. The previous post I linked (post #2 here) and the posts it links to has much more information about safe, natural, organic, “chemical free”, and “green” mattresses and mattress materials. If “natural” is important to you for reasons other than “safety” then some examples of all or “mostly” natural mattress materials would include natural fibers (cotton, wool, linen etc), 100% natural latex, and steel (although in strict terms it’s not “natural”).
I’m always happy to help with “how” to choose, act as a fact check, help with any specific questions you may have, and to help you identify any lower quality materials or “weak links” in a mattress relative to body type, budget range, and circumstances.so that you can make more meaningful comparisons between mattresses but I don’t make specific recommendations for materials (which is a preference choice not a “better/worse” choice) or specific manufacturers or mattresses because there are too many unknowns, variables and individual preferences and criteria involved. Outside of PPP … a mattress is only as good as it’s construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label.
While I do recommend the members here “as a group”, there are also many other retailers/manufacturers across the country that are transparent about the materials they use in their mattresses and make good quality/value mattresses that aren’t members of the site (at least yet) that may also be well worth considering if they compare well to the other finalists you are looking at.
Once you are down to finalists that are all choices between “good and good” and none of them have any weak links or lower quality materials in their design and if there are no clear winners between them then you are in the fortunate position that any of them would likely be a suitable choice and post #2 here can help you make a final choice based on your material preferences, your conversations with each of them, their prices, the return/exchange options they have, any additional extras that are part of each purchase, and on “informed best judgement” based on all the other objective, subjective, and intangible parts of your personal value equation that are most important to you.
Phoenix