joshi,
Never owned a 12 fret, but have tried them. Would recommend trying a cut-away version too. Collectively, what the guys wrote about a 12 fret seems to be not only generally held opinions, but I don't remember reading any dissent about anything they wrote.
Best advice I can offer is play as many as you can. One may say "Take me home tonight" ("Just like Ronnie says"). Not all Grand Concerts are as expensive as an 812, and I think you are correct about buying used if available. I own two 14 fret Grand Concerts, both bought used after playing each in the store. I have actually played quite a few different ones over the years.
The 322 Mahogany over Taz Blackwood was mentioned a couple of times. Prior to 2016 (if I have the right year), the 322 was Spruce over Sapele only, so look before you leap. I always wanted to play the Blackwood model, but the only authorized Taylor dealer in Boston is Guitar Center. Their policy on many expensive guitars is an on-line type of sale only, deliverable to a store. I don't want to buy an expensive instrument I haven't played, so I never got to play one. Nor do I want to buy into such an outrageous business model. I had asked both Boston Guitar Centers about getting one in to try, and was refused by both stores. Actually I asked for either a 322 or a 324. I just wanted to hear/play one with those woods.
Also, not all Grand Concerts are short scale. My 412e-R is short scale, but my 512ce has a 25.5" scale. Called Taylor about this once, and they said they switched standard GC scales somewhere in the mid-2000s.
You don't show where you are located, but if you are in the Boston / New England area and are looking for good used guitars, Try contacting Mr. Music on Harvard St in the west Boston suburb of Allston, and see what they have in stock at the moment. I bought both my GCs there used, and they were, and still are, in near mint condition.
Lastly, I applaud your search for a Grand Concert. Turns out that my short scale 412e-R has become a personal favorite and gets a lot of playing time. I string it with Elixir PB HD Lights. While these are great sounding guitars, they are not meant to be as loud as larger models. Mine do not sound as good if I play them too harshly.
Don