We have a 15 year old firm Marshall pocket coil king size mattress. I am thinking about getting a 20 ILD latex topper to try the “feel” of latex before springing for a layered latex mattress. I am a 300 lb guy and wondering if the 2" 20 ILD topper would be sufficient as a pressure point reliever or whether the 3" 20 ILD topper from SleeponLatex might be a better choice. Another option could be the 2" 30 ILD, but am hesitant as to whether this might be too firm for pressure relief.
I provide some caution as to drawing a conclusion by placing a latex topper on a 15 year old mattress and attempting to use that as a predictor of what an all-latex mattress might feel like. Besides the fact that your older mattress had gone through quite a bit of wear, the componentry used (polyfoam and springs) would be quite different from an all-latex configuration, as all of the layers of a mattress work together to provide overall comfort.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
If you need a little to a fair bit of extra softness and pressure relief … then a 2" topper would be in the average range. If you need a fair bit to a lot of extra softness then a 3" topper would probably be the best choice.
Heavier weights generally do better with firmer and thicker toppers. Lighter weights generally do best with softer and thinner toppers.
In general … 19 - 24 ILD in latex would be considered to be in the soft range for “average” body types while very light body types may be OK with an ultra soft 14 ILD and heavier body types would probably do better with 28 ILD and above.
Overall, you’d probably be better served testing out an all-latex mattress in person (if possible) as a more reliable indicator of what an all-latex mattress would feel like versus placing a latex topper on an older innerspring mattress.