Hi natinaut,
It would be reasonable to expect that any mattress that meets the guidelines here and was well inside the range of comfort/support that was suitable for you in terms of PPP and wasn’t on the edge of being too soft for you initially (see post #2 here) would have a useful life that would meet your 5 - 10 year criteria (and most likely closer to 10 years than 5).
Post #2 here includes some latex/innerspring hybrids that may be of interest but most (although not all) of them would be in the upper end of your budget range or higher.
Posts #3 and #4 here also include some of the lower budget latex and latex hybrid mattresses that I’m aware of.
There are also some other types of lower budget mattresses listed in post #4 here as well.
You can see some comments about “one firmness fits all” mattresses that includes Tuft & Needle and Casper in post #3 here and the rest of the topic. A forum search on casper tuft needle (you can just click the link) will also bring up all the forum posts that mention both of them.
The Dreamfoam Ultimate Dreams latex is a little different than the other two you are considering because they use Talalay latex in the comfort layer (which is a higher quality material) and they also allow you to customize the firmness level of your mattress as well. They also have other options (such as the Eurotop) that allow for comfort layer exchanges so you can exchange the comfort layer in your mattress for a firmer or softer version without having to replace the entire mattress if your sleeping experience indicates that you need to.
Although this can be a good strategy if you can test the mattress/topper combination in person before a purchase … you can see my comments about choosing a mattress with the intention of adding a topper in post #2 here. In general I would avoid this approach because of the uncertainty involved with two purchase choices instead of only one and choosing a topper that would be suitable in terms of PPP for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper(again unless you can test the combination in person) and then use the option to add a topper as a “backup” strategy in case your choice doesn’t turn out as well as you hoped for rather than a “primary” strategy.
It’s unlikely that this would sleep hot for most people with the top layer of latex because latex is generally more breathable than other types of foam. Most sleeping temperature issues would be with either memory foam mattresses or with mattresses that are in a softer range where you are sleeping more directly on the foam materials because any type of foam will be less breathable and more insulating than natural or synthetic fibers that are sometimes used in quilting layers. It would also depend on many other factors as well including where you are in the “oven to iceberg” range. There is more about the many variables that can affect temperature regulation (including the type of mattress protector you use along with your sheets and bedding) in post #2 here.
I’m not sure which reviews you are referring to and most of the neutral or negative reviews about the Ultimate Dreams Natural talk about the firmness/softness of the Dunlop latex being different from what they expected (which is always subjective and relative to the body type, sleeping positions, perceptions, or preferences of each person) or are from people that didn’t read the description of the mattress (they somehow thought that this was an “all latex” mattress) but regardless of whether it is Dunlop or Talalay or uses natural or synthetic latex or a blend of both, latex is one of the most durable materials in the industry and wouldn’t have any significant issues with sagging unless a mattress included defective materials or unless the support system under the mattress wasn’t suitable.
At your weight … the 1.5 lb firmer polyfoam support layer in the UD latex hybrid (with the Talalay comfort layer) wouldn’t be a durability issue.
I would also be very cautious about using mattress reviews (positive or negative) as a reliable source of mattress research or information and in many cases they can be much more misleading than helpful (see post #13 here).
There is more about the different ways to choose a mattress that is most suitable for you (either online or local) and how to minimize the risks involved in each of them in post #2 here.
Post #13 here also has more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses and help you choose between your “finalists”.
Phoenix