Get base Debian, you’ll have more options for desktop environment. Once you get past the installation hassle it should just work for the rest of times. MX has its place but it’s specifically made to have no systemd which may not be something a new user is looking for. It feels very opinionated, is what I’m trying to say. May be your thing of course, but I’d recommend reading more on its philosophy before picking.

8 years is probably not old enough to require lighter desktops if the machines were at least mid range at the time. You should be able to use gnome or KDE as you please. Nothing against XFCE in principle, but it can be a little clunky especially for a laptop. No touch gestures, for example.