Hi!
I haven't yet decided what kind of display I will use, but here's few questions I need answered if I end up with a TV.
1. If I buy a TV with scart compatible with RGB, does it automatically also mean it is compatible with 60Hz? Or does it even need to be compatible with 60Hz?
If the TV is rated PAL it is not required to accept 60Hz signals. It is my experience that most SCART sockets accept 60Hz RGB fine though despite being rated as a PAL set.
2. As it's quite impossible to know if a TV has RGB scart or not when buying used, can you give me advice what brands generally have/have not rgb scarts? I'd probably end up buying some TV from 90's.
Most 25inch sets from the last 10 years have had SCART sockets. However, having a SCART socket does not automatically means that it will accept RGB. The only mandatory signal through SCART is audio and composite video. Major brands should be fine though: Sony, Philips, JVC and Grundig. Avoid off-brand stuff like ProVision, AudioSonic, Panasonic, etc.
3. Just so I'm not assuming anything wrong, if I connect the TV to arcadevga with a proper cable, _everything_ will run properly?
If _everything_ means horizontal oriented 4:3 15kHz standard resolution games, then yes. Vertical oriented games can often not be displayed on a horizontal screen without scaling.
Also, a TV cannot display 25kHz medium resolution games without interlace or scaling. The amount of medium resolution games supported by MAME is expected to increase of the coming years. We are now seeing Sega System 24, Sega Model X and Atari Vegas hardware emulated which require medium resolution monitors.
In case you are wondering: scaling is bad because it degrades picture quality and authenticity. Interlace is bad because it makes the screen flicker and is obviously not authentic either. Interlace puts a lot of strain on the eyes as well. Not recommended.
4.Off-topic question, if I end up buying a PC monitor, will it need to be a high quality one? Or will some used multisync 21" from mid nineties do the job?
Most likely, yes. You don't need very high resolution monitors to run arcade games. If you get something that has at least 500 - 600 lines you will be fine.