OK, you actually will get the exact same late 2018 Mac Mini that I have, except for the additional storage. As I mentioned, mine has a 256 gig SSD.
At least for me, the machine works real well, easily satisfies my "need for speed", and is rock solid. Previously, the late 2012 Mac Mini I owned had a Core i7 processor, and I felt that possibly I would see a drop off in performance. But that's not the case it all. Very, very pleased with mine.
When I purchased it back in February from Best Buy, it came with Mojave. But since I do my backups with SuperDuper!, it was a piece of cake to do a clean, fresh installation of Catalina, and then migrate/copy all my files, folders, apps, settings, etc. from that backup. As I mentioned, I am now on V10.15.7 of Catalina, and it works fine.
As for backups, since you only need to do it once a week, I can easily, and without any hesitation, recommend SuperDuper!:
Just click on where it says "Download Now!" (there's a linkable word download below that). The free version works fine, but I actually purchased it after using it free for about a year, mainly to support Dave Nanian, the developer of it. (I actually have never used any of the additional features that come with the paid version).
It is a solid, reliable program, and I've been using it for more than 4 years. Right now, as you can see, that latest version is compatible with the last 6 versions of the Mac OS, including Catalina (and Mojave). Besides being rock solid, the beauty of it is that such backups are
bootable, that is, you can boot you machine from it. That one feature alone makes it so worthwhile! I cannot tell you how easy it makes it to do a clean, fresh installation of either a new Mac OS, or "within" the same Mac OS.
Also, when I do the weekly back up for each of my Macs, I just let it replace the one from the prior week. Always has worked for me.
Compatibility with Big Sur will, I am certain, come soon. But it has been challenging for Dave (there is a similar program called Carbon Copy Cloner that has the same features. It is, though, not free, and even updates cost money. SuperDuper! has
always been free, including updates).
As for your external drives, how much space are you using on the 4 TB Passport device? Also, when you get your Mac Mini, do you plan on partitioning it, or will you just leave it as is, ie, have all of the "less than 1 TB" space available for your processing?
Finally, as for 32 bit versus 64 bit software, I believe Office 365 is 64-bit. I use LIbreOffice, and it is a solid performer also. The equivalent Word (called Writer Document) and Excel (called Calc Spreadsheet) modules are easy to use, and have more than enough compatible format abilities to read and write equivalent Word and Excel formats. It does not have an EMail client (Outlook) like the various Office suites have. I used to use Office 2016, but Outlook 2016 really "screwed" me once, and it was at that time when I switched to LibreOffice, and also the excellent freeware EMail client Thunderbird. I am a "happy camper" with that setup.
As for Apple's Mail program, hopefully it works fine for you. Myself, no matter which Mac discussion sites I visit (I visit two others besides Mac-Help), I consistently see users have issues with Apple's Mail program. And it happens with any version. Hence, I have always avoided it. Same with Safari. I use Brave, although Firefox (which I previously used) seems to be getting faster and faster.
Whew! OK, that's enough for now. Sorry for the long post. And best of all, congratulations! You will really like your new Mac Mini.