Hi Connor,
Just in case you haven’t read it yet … the first place I would start is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you with how to make the best possible choice … and know how and why to avoid the worst ones.
Unfortunately … there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict which mattress will be the best match for you or the materials you would prefer based on a formula or specs (either yours or a mattress) or “theory at a distance” and there really isn’t anything that can replace your own personal experience (see mattress firmness/comfort levels in post #2 here). I would avoid trying to decide on a mattress based on specs or “theory” because it won’t give you any reference points or personal experience about how all the numbers and information “add up” and will actually “feel” for you.
If you let me know your city or zip code I’d be happy to let you know about any of the better options or possibilities I’m aware of in your area and the tutorial post includes links to the better online options I’m aware of (in the optional online step) if you decide to make an online purchase although I would strongly encourage you to make sure you have some familiarity with different types of mattresses and materials because only one or two mattresses aren’t a representative sample of the range of mattresses that are available in any particular mattress category or with different materials.
If you do decide to make an online purchase that either uses materials you aren’t familiar with or where you aren’t confident that the mattress will be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then I would make sure you know and are comfortable with the options you have after a purchase to either exchange the mattress (or in some cases individual layers) or return the mattress which may become a more important part of your personal value equation.
A 1.5 lb polyfoam base layer would certainly be suitable for your weight in this budget range and the support core isn’t usually the weak link of a mattress anyway (see the guidelines here). There is also more detailed information about the many variables that can affect durability and the useful life of a mattress relative to each person in post #4 here.
Phoenix