Back up your system is great advice.
There are many ways to do it.
Find one, pick one.
Clonezilla was mentioned; that's fine.
Simply copying the entire system to external media occasionally is important, regardless of the methods used.
Personally I usually use something like the dd command; it can copy bits and bytes to nearly anything so that's what I do.
MX Linux is definitely a very good choice, and recent versions now offer a choice of 3 different environments: the default distribution comes with a nicely configured Xfce desktop; it's a good overall choice. Those who miss the old MEPIS that preceded MX Linux might appreciate their next offering, MX Linux KDE. Finally, MX Linux added a setup with Fluxbox. Note that you can, on your own, add other window managers and create your own variation as well.
I also happen to like a "relative" of MX Linux that came out not long after MEPIS was introduced. It's called antiX and it rivals Puppy Linux and practically anything else for running effectively on older hardware. There are people actually using systems that are 15-20 years old and having success, as long as their hardware continues to work. I am using MX Linux now, but I will be booting one of my two installed instances of antiX momentarily. I have MANY USB versions and even older ones from CD that I still have and occasionally use. I've been successful getting more than one old piece of someone else's "cast off" to work quite well with either MX Linux or antiX!