Hi minpat,
You are certainly making a great quality/value choice and I hope that your guests appreciate that they will be sleeping on a higher quality and more durable mattress than the Sealy mattress you are using yourself. Congratulations on your new mattress 
I’m not sure you are clear between the difference between a box spring and a foundation. A box spring has springs inside it that flex under the mattress and there are very few one sided mattresses today that use them as a support system and in many cases they will invalidate a mattress warranty. Most mattresses in the industry today need a steel or wooden bedframe with a foundation that has minimal to no flex (vs a box spring that flexes) or a platform bed which also has little to no flex under the mattress. There are many in the industry that mix up the terminology between box springs and foundations even though they are very different products.
There is more information about the different support systems (bedframes and foundations or platform beds) that are generally suitable for different types of mattresses and some examples of each of them in post #1 here.
An all latex mattress will generally do best with a firm, flat, and evenly supportive support surface underneath it that has minimal to no flex under the mattress and for larger sizes with at least one center support beam that has good support to the floor to prevent any sagging in the middle of the mattress. The components need to be strong and durable enough to support the weight of the mattress and the people sleeping on it without some of the parts bending, sagging, or breaking over time. The support surface under the mattress should have enough surface area to prevent the mattress from sagging through any gaps or spaces in the support surface over time but still allow some airflow under the mattress. I would suggest that in a slatted support system (either a foundation on a steel or wooden bedframe or a platform bed with a slatted support surface) that any gaps between the slats are no more than 3" (with 1 x 3 slats) although less than that would be better yet.
I hope so as well.
If a mattress is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know whether a specific mattress/topper combination is a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) is based on your own careful testing or personal experience on the mattress/topper combination. If you can’t test the combination in person then there is always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
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 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. It also includes a link to a list of some of the better online sources for toppers I’m aware of (including Sleep On Latex) and a link to the online suppliers that have good exchange/return policies as well (just in case your choice doesn’t turn out as well as you hoped for).
If you do end up making a suitable topper choice and the mattress/topper combination turns out to be a good “match” for you in terms of PPP then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers or a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
I’m looking forward to your comments and feedback both about your mattress and your topper once you’ve received them.
Phoenix