H ibeda.
Welcome to our Mattress Forum! 
Although I do personally like the feel and properties of latex, the choice of materials is a personal preference and there are many people that prefer a Memory Foam feel and are “used” to it to such a degree that any change to a different material may be perceived as very uncomfortable. There is absolutely nothing wrong using Memory Foam in a mattress… provided that it meets durability guidelines here . My cautions on the site are related to most of the brand names who use less durable materials in their mattresses than most of their smaller competitors, or some of the many online companies who ship compressed “bed in a box” mattresses that are using lower quality materials in their products, which would compromise the durability and useful life of a mattress. Of course part of your perception maybe also that memory foam itself has a unique feel and is a more polarizing choice (people tend to really like or dislike it). When it comes to Memory Foam in fact many of our Trusted Members here (that I think very highly of and I believe that they compete well with best in the industry in terms of their quality, value, service, knowledge, and transparency) carry high quality memory foam products.
If you like the feel of Memory Foam, then choosing the “best possible” mattress for you would come down to FIRST finding a few knowledgeable and transparent retailers and/or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell. When assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
While Memory foam does a very good job at minimizing motions transfer, it will generally be the least breathable and most insulating of the foam comfort materials, so as you noted temperature can be an issue and you’d want to make sure that you don’t choose something with too much memory foam on top that you sink in too deeply as the more you sink into it the more will sleep hot.
It’s difficult to quantify with any accuracy the sleeping temperature of a mattress for any particular person because there are too many variables involved including the type of mattress protector and the sheets and bedding that you use . But of course I would guess that t the main culprit is the Memory foam in the comfort layers of your mattress and how deep of a cradle this forms around your body. You can read 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. In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material.
There are the new generation of gel memory foams with different types and chemical formulations that may be sleeping somewhat cooler and also can have a faster response time than others but all memory foam has a very low resilience (springiness). Some types of memory foam are also more breathable than others and have less effect on sleeping temperature but memory foam, in general, tends to sleep warmer for some people than other types of foam materials (latex foam or polyfoam). There is more about the different properties that can be formulated into memory foam in post #9 here and post #8 here.
In addition to the information above, this post … post #29 here has more information about temperature regulation and the microclimate on a mattress.
Memory Foam is very good at isolating movement between sleeping partners because of its ability to respond to pressure in a very local area. It shares this quality with latex and to a lesser degree some higher quality polyfoam. Keep in mind that sometimes what people perceive as motion transfer may be due to the deeper layers within the mattress that respond to the movements from the layers above. For example … memory foam is a much more energy absorbing material and is also very point elastic (conforms to the shape of the body more exactly … at least if it’s good memory foam) and would typically be the “best” at motion isolation. But if you put memory foam on top of an innerspring that has helicals then you would still feel the stronger or faster movements that went “through” the memory foam and compressed the springs. In the same way, if you had a memory foam comfort layer and then had a quilted cover that didn’t have any stretch you may feel smaller movements more because the cover would “pull” when you moved … especially if someone was heavier.
There is also more information in post #2 here that talks about the various symptoms that people may experience on a mattress and some of the reasons that can cause them that can help with the detective work or trial and error that may be necessary to identify the types of changes that may be helpful.
It is up to the manufacturer themselves to decide whether they include in the price of their mattresses the cost of any future returns. The trend now, especially with the online eCommerce is to offer free return/free exchanges for an average period of 90/100 days and allow for in-home trial and ensure suitability of the system with sleeper.
So-called “free” shipping or “free” exchanges could be also just about marketing because the cost of returning or exchanging a mattress or individual layers are all built in to the purchase price of a product that offers “free shipping” or “free returns” so the majority of people who don’t return or exchange a mattress are the ones who pay for the minority that do. You can read more about exchange and return policies in post #25 here. Some manufacturers or retailers include the “real” return and/or exchange costs into the cost of their product and some manufacturers or retailers prefer to have them as a separate cost so only the people that return or exchange a mattress or a layer pay for it.
I hope this gives you a good start in the search for “your” mattress and I or any other Expert Member of the site would be glad assist you with any questions that you may have.
Phoenix