Are you sure that this use of "emunt" is proper Latin? I ask because in the Romance languages that I know, you don't "buy" a lunch (or a dinner or a breakfast). You can buy the food to cook a lunch, or yoy can pay for a lunch, if it is already prepared, in a restaurant or something, but you don't "buy" a lunch.
You can absolutely buy a meal in Spanish and Portuguese.
Here is just one example.
There's nothing improper about buying a meal in Romance languages, so I have no reason to believe the Latin is wrong either.
Well, at least in Spain, where I was born and I live, "voy a comprar la cena" (I am going to buy the dinner) can mean 1) I am going to buy the things that I need to prepare the dinner or 2) I am going to a place where they sell prepared meals, like in your example, some kind of take-away shop, and I going to pick up the prepared meal and bring it home, where I'll eat it. Considering the context (a Roman soldier) I think none of them is very likely. When it comes to a Roman soldier, I guess that the most likely assumption is that he is going to some kind of "taberna" to pay for a meal in order to eat it there (the equivalent of a modern-day restaurant or pub). Hence my doubt. In Spanish (and I think it is pretty much the same in Catalan, French or Italian) you don't say "to buy the dinner" if you're going to eat it in the same place where they sell it.
On a side note, English has a tendency to use "buy" with food and drink in cases where most Romance languages don't use it. For instance, it is pretty usual to say in English "I'll buy you a drink" meaning that you want to pay for the drink someone else is having in a pub or bar, etc. In this situation, you cannot say in Spanish "Te compro una copa", that's totally weird. In this case, you would say "Te invito a una copa".
Was "sanus" really used like this to describe things that deliver good health (i.e. the way the English word "healthful" is more properly used today)?
I believe I've seen cibus sanus (healthy food) before, but I'll have to look for a reference.
And I wouldn't say healthful is more properly used for this. Healthy has a wider definition than it once did.
It's quite proper to speak of kale being a healthy food.
I think this is a fair question. "Healthy" certainly has a broader range of meaning now (no arguments there, kale is a healthy food), but a major part of speaking another language is learning their idiomatic way of saying things. That's the difference between translation by decoding, and learning to speak the other language itself. If you don't learn the Spanish or Portuguese idioms, you'll look fairly foolish trying to speak the language, and you'll look fairly arrogant saying you're speaking it correctly (when you're... not.) It might be difficult because there's a lot of latin to search through (even without medieval latin). I for one wouldn't mind finding whatever common idiom expresses this same idea (a lunch that is good and healthy). Maybe something within the range of "Salus, salutis, f" fits better idiomatically?