Hi jisungk,
The first place to start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and avoid the worst ones.
You’re right that latex is the most breathable of all the foam materials but there are also many other factors that can affect the sleeping temperature of a mattress that are talked about in post #2 here. Firmness is subjective and relative to each person so you will need to use your own personal testing or experience to decide which mattress is firm enough for you and is the best “match” for your body type and sleeping positions in terms of PPP (see mattress firmness/comfort levels in post #2 here). If you are looking at an online purchase that you can’t test ahead of time then you would need more detailed conversations with each online retailer or manufacturer to help you decide which of the mattresses they carry was the best “match” for you in terms of firmness and your personal value equation.
You can read more of my thoughts about BedinaBox in post #2 here. I would be very cautious with any memory foam mattress that didn’t meet the guidelines listed in post #10 here.
Dreamfoam is one of the members of this site which means that I believe they are one of the manufacturers that competes well with the best in the industry in terms of quality and value.
Phoenix