I installed an SSD on a desktop from 2007 and it was definitely worth it. Even with a slow SATA 1 controller, the machine runs so much better. A 256 GB SSD can be had for $25 Canadian these days. In my view it simply isn't worth running any PC without an SSD boot drive. Your mileage may vary of course. MX-Linux is based on Debian.

My laptop's not worth upgrading to an SSD, it was new in 2009 and it's battery no longer holds a charge. I only use it on weekends when I'm up late gaming so I can read messages, look up things, watch youtube, etc. without having to get up and pause my games to go to my desktop in another room. There's no data on my laptop that I can't afford to lose, it's all backed up or copies are maintained on my main PC with network drive backups. I'm planning to stick with something Debian based, and was thinking of going with Mint XFCE because after I install Mint on my parent's PC, it might be helpful to have Mint system here to get more familiar with. I've never tried MX Linux before so I also considered that just to see what it's like. If you're curious, my laptop has a 2.1 GHz dual-core Intel and 4 GB RAM. I can't remember how big the HDD is from memory, but enough space for both Xubuntu and Windows 10 with storage to spare. It originally came with Windows 7 and I got the free upgrade to 10. I maintain 10 on another partition because once in a while I come across something I can't do in Linux (Such as, I have a third party Nintendo Switch controller, and when Switch 2 compatibility firmware was released, they don't support Linux so I needed Windows to do the update.) Once 10 support is over, it probably won't be worth it to keep since eventually any Windows-only tasks won't support 10.   I like to stick to XFCE, I figure even if a system can handle more, over time as it ages it's proven reliable in the long run. I had an old desktop (originally ran Windows XP) that ran Xubuntu until the hardware died and couldn't start up anymore. I do dabble in other desktop environments from time to time. I have cinnamon installed on my main desktop (Xubuntu) and every now and then I feel like a different flavor on my computer for the day. Long ago I bought a junk PC from a local shop for the only purpose of experimenting with Linux and I tried lighter environments like Lubuntu (which used LXDE at the time.)

Neat

If your old laptop has a conventional HDD, an SSD will supercharge it regardless of what distro you install. Looks like you are committed to Xfce and in my view it has the best compromise between ease of use and lightness of resources. There are lighter desktops for sure and certainly nicer looking ones but I always keep coming back to Xfce. Linux Mint's Xfce version looks a lot like its flagship Cinnamon edition. Xubuntu you already know. MX-Linux has a slightly different look and feel out of the box, but it has a really excellent collection of graphical tools as part of the package. If you want more cutting edge you could try installing EndeavourOS which has a very slick Xfce desktop and is based on Arch. Of course any distro can be configured to look like any other. I think you might enjoy MX-Linux. It'll look a bit different so I wouldn't recommend it for your parents, but you should at least check it out as a USB installation.