1. Is the best connectivity option for a CRT TV still Scart?
2. What is the easiest method to connect a video card to Scart? Will any VGA to Scart converter work?
3. Will all CRT TV's with a Scart input do 15khz?
4. Any reason not to go for a flatscreen CRT TV?
1. Won't hurt, lots of SACRT cables and accessories still available. SCART is a good choice, if you can get an RGB CRT TV that has it.
2.
https://www.aussiearcade.com/showthread.php/87668-A-guide-to-connecting-your-Windows-PC-to-an-SD-CRT-TV-PVM-or-Arcade-Monitor With the c-sync feature in crt_emudriver, all you need is a VGA to SCART cable that would work with a projector.
3. Even assuming you mean CRT TVs only, the answer is still no. As Haynor noted, all ED/HD/widescreen consumer CRTs will upscale 15kHz. You want an older SD CRT TV with RGB SCART.
4. Geometry can be tricky on flat CRTs. May also not look quite right when playing on an -older-style cab. I like the curve, myself.
And yes, a good SCART TV will be very nice, nothing to complain about. If you can find a nice Nanao 29" monitor (~$600) they're awesome, as you can overdrive their chassis to make them really bright. And an arcade monitor will probably sync to a slightly wider range (15.5kHz to 16.2kHz) than a CRT TV, though there won't be much in it. Other than rolls-royce arcade-monitor examples though, a nice NEC/Panasonic/JVC/Phillips/Loewe SCART CRT TV should be just as good.