Yeah, purchase decisions are difficult

. I used to be in the same situation with T12 stations. While I can't answer all your question, I can answer some of them.
1. Version 2.1 vs version 3. Rumors say KSGER's v2.1 stations run outsorced firmware for which the seller pays something. Version 3 is the same hardware, but with a completely rewritten firmware to avoid paying to the software developer. It's hard to find a good comparison, but people say firmwares are mostly identical, but 2.1S was more polished (as of a few months ago).
2. Is updatable firmware really needed? I have 2.1 station that "just works". I don't feel like I want to replace/upgrade firmware.
So, my advice: go for proven 2.1S with a shake sensor. The sensor works really great on my station, it's not like an encapsulated ball with spring contacts, it really detects motion (I'd say it's even too sensitive). I bought a complete kit in metal enclosure and I'm happy with it. It's really cool (or "hot"?), fast and compact. Beware it doesn't come with handpiece stand.
Just check tip ground connection when it arrives, I heard someone had issues with this and for some reason (bad SMPS?) the tip was at ~115V potential. So, worth checking.