Can I swap a SSD into my late-2014 mac-mini to replace failed HDD?

I've been trying to configure my late 2014 mac mini (8 GB, 1 TB HDD) to allow my wife to work on a mac at home since her old MacBook Air just died. Previously I'd use the mini as a media computer and a Plex server. Anyway, I got it all set up for her, when I noticed the HDD was booting slowly. So I ran First Aid, then First Aid from recovery mode, which both failed. Now I can only boot the machine into recovery mode or safe mode. If I try a normal boot, it stops about 70% through the normal cycle and turns itself off.

So I figure the hard drive is fried. That's two Apple failures in two weeks, machines that have been reliable for years (sigh, are we sure Apples can't get COVID-19???).

I'm trying to avoid buying a brand new machine, and I figure this chassis with a 1 TB SSD would run quite well for what I need, which isn't very intense. I've watched a couple of youTube videos about performing the upgrade. There seems to be a fairly simple upgrade, if you purchase a special cable, to install a PCIE ssd. But it's unclear to me whether that works on a HDD-only mini (Is that why you need to buy the cable, or is there more to it?) and whether the PCIE can be the primary (and only) boot drive.

I've also seen videos about installing a 1 TB SATA SSD to replace the the HDD entirely. It requires completely disassembling the mini and reassembling it. While I'm reasonably handy with this sort of stuff, my confidence of pulling this off is maybe 70%.

Another complicating factor is that I want to run MS Office 2011, which means I should install Sierra or HIgh Sierra at the latest. Third factor is that because this was never a "mission critical" platform for me, I was never running Time Machine. So I'm going to need to install the boot drive from scratch.

OK, so here are my questions.

1. Does anyone have experience upgrading an original HDD mac mini with a PCIE drive as the boot drive? Does it work?
2. If that's not an option, is the replacement of the SATA HDD as bad as it looks?
3. Are there hardware vendors out there that provide good support for this kind of operation?
4. Any advice on whether I should install Sierra or High Sierra given that running Office 2011 faultlessly is my wife's most important requirement?.
5. Tips on where to find an install-able image of whichever version I need to install?

Thank you in advance for your help.

Michael