Hi Davchun,
Welcome to the Mattress Forum! 
A five pound density memory foam would be a good quality material, but almost all memory foam would be considered quite plush (usually in the mid-teen ILD range). There are of course differences in response rates and sensitivity to heat that can manipulate the feel, but overall memory foam is a material with low resiliency and high hysteresis and generally wouldn’t be considered by most as “medium-firm”. Also, density and plushness aren’t necessarily correlated with memory foam.
Latex on the other hand does come in many different ILD ranges and can be found in what most people would consider a “medium-firm” (most likely mid-20s to low 30s ILD) range. Density and softness have an almost direct relationship with latex. Talalay latex will tend to be described mostly by ILD, with a mid-20s ILD being in the mid to upper 3 lb to low 4 lb density range for a Talalay Global blended Talalay, as an example. Dunlop latex is more accurately described via density, although many brands will convert that to an approximate ILD range. Depending upon the rand and blend, a 5 lb density would tend to be in an upper 20 – lower 30 ILD range, but this can vary a bit.
As for breathability, Talalay latex will be the most breathable foam on your list, followed by Dunlop latex and then memory foam. There are some newer generation of memory foams designed to be more breathable.
As for our overall temperature, it’s not really possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range. Having a breathable cover is certainly important, but I would also pay very close attention to the type of foam you choose for your topper.
If you can’t test the combination in person then there will always be 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.
It is always good idea when choosing a suitable topper to take into account the mattress it will be placed on as these both work hand in hand to give you the pressure relief and posture alignment you need and you will need to choose the thickness and softness that has the best odds of working well for you.
Assuming that the mattress on which the topper will be placed is in good condition and has a reasonably flat and evenly supportive surface (no significant impressions or soft spots) then a topper can work very well to improve pressure relief and what I call “secondary support” (the type of support that fills in the gaps in a sleeping profile and is connected to the comfort layers rather than the support layers of a 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 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. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
Keeping in mind that most of the latex or memory foam toppers that are offered are coming in 2"- 4" thickness, here are some of [url= https://mattressunderground.com/the-underground/our-trusted-members.html]our trusted members[/url] who offer latex toppers that could fit your criteria.
Sleep EZ has 2" or 3" NR Talalay & Dunlop toppers
Arizona Premium 2" or 3" Talalay & Dunlop toppers
Foam Sweet Foam Has 3" toppers both Talalay and Dunlop
Latex Mattress Factory 2" or 3" toppers
My Green Mattress Dunlop topper
Rocky Mountains has 2"-4" NR Dunlop
Sedona Sleep a 3" Talalay blended Latex topper with zippered bamboo cover http://sedonasleep.com/talalay-latex-mattress-topper/
Soaring Heart
Soaring Heart 2 organic Wool mattress toppers 2" 100% organic NR Dunlop latex wrapped in 1"of premium organic wool and hand-tufted to insure long-lasting comfort + A few options of Wool Toppers + 3" NR Latex encased in organic cotton knit fabric. (Pillow Top)
I hope this helps and let me know if you have more questions and I’ll do my best to assist you.
Phoenix