the big one: how do you choose speakers? By what features, data?

Two cents:   Try and figure out what type of speaker you like the sound of first. That really narrows down the list and ought to be the easiest thing to listen for. Our choices in design can change over time; that's fine as one upgrades.

Finding the 'house' sound that you prefer, of a good brand, within the type of loudspeaker, is worthwhile next (eg B&W, Focal, Dynaudio, Sonus Faber, et al). Finding a good used (within budget) pair of speakers as high up the line is great.

I am 100% into used. Most are taken care of very well, especially the higher up. As some say, these days, 'watts is cheap'. There are great amps with plenty of power, AB or D that don't break the bank. I have never allowed sensitivity to dictate my speaker choice and sound but I do usually dislike horn in-your-face sound so it never mattered. I did gradually move from electrostats to boxes.

Buying or upgrading speakers is always an interesting journey. Good work on doing your homework as best you can!

Haven't thought much in depth about specs and the sound of a speaker but I doubt that can be used as a workable guide. Speakers can measure similarly and sound vastly different.  

I don't have the music-speak for it, but to me (after 45 years of listening) it is all about how the speaker 'releases the notes'. Some may sound 'lightweight' and still measure like fat or dull-sounding speakers.  And, sadly, not all speaker lines have guaranteed consistent sounding models...but most seem to have their own sound.

I'd say try to hear as much as you can and take notes on general sound quality. Amps can affect this somewhat (might not hurt to note the amp you heard driving the speaker you auditioned). Buying an unheard speaker is a lot larger crapshoot than amplification or source. I only bought unheard, used D2s because I knew exactly what D1s sounded like in my room and knew the only difference was the additional bottom end. That was not a gamble.  :-)  

I'd suggest reading reviews (and lots and lots) as a better way than specs to arrive at potential speakers to try to listen to. It takes time but when you start to understand different reviewers' biases it gets more informative. Specs can sometimes tell you what to stay away from.    Have at it!