Probably my 2 slot CMVS. Most everything else I have cost money, while the CMVS cost a lot of time getting it just right, and was something I had wanted to build for over a decade. Ironically, I haven't actually played it much.
If you need to buy something I'd just get a USBC 3.whatever flash drive. I just used mini SD cards since that's what I had laying around, but the cheap Samsung cards weren't fast enough. If you used a mini SD a name brand A1 or faster card would be fine.
I've swapped the drive, it's pretty easy. Make sure you are using a quality screwdriver though, those are some tiny ass screws I wouldn't want to strip.
Reinstalling SteamOS was a little tricky for me. It kept getting stuck like it was taking forever to load but never freezing or giving...
Because it's a really good game? There is a lot of padding though, I bet if you didn't do the side stuff it would be a more normal 35 hour or so game. But the side stuff is mostly good and I actually want to play it.