I’d echo paul about fit being crucial.
The other thing to note is that with modern integrated TT bikes, it’s much harder to adjust the position significantly using different stem, bars etc. so you really need to sort out your fit before you start buying bikes (assuming you want an integrated bike and just want to buy one!).
On the terminology front:
Stack and Reach are measurements that you can do on any bike - they are the vertical distance from the BB to the top of the head tube and the horizontal distance respectively. I’d suggest they might be more useful for road bike fitting than TT bike, as TT bike seat angles can be quite different, and some offer a large adjustment factor in the seat clamp.
The long+low or tall+narrow stuff you see coming up are essentially different philosophies on how to best position the rider to minimise drag. To a certain extent, the best approach depends on the body shape, flexibility and strength of the rider. Long and low used to be dominant, and some of those bikes are still really good, but you should probably just go find a fitter who really understands TT (tri fitters are likely to be good but might optimise a bit more for comfort) and take his advice.