Hi Villager,
Welcome to the Mattress Forum! 
Adding a topper to a few-year-old mattress can be an effective way to create an appropriate comfort on a mattress that is still in good shape and supportive – just uncomfortable. And using the appropriate type of material (like the latex you’re considering from SleepEZ) can also help a bit with avoiding some of the “temperature issues” some people are sensitive to with memory foam.
The topper thread in post #2 here has some excellent advice about selecting a topper that should provide the guidance you need in this situation. You may also wish to phone SleepEZ (a site member here, which means that I think highly of them) for advice on an appropriate plushness, or at least feedback on what options you are considering.
Phoenix