The One Demon wrote on Yesterday, 21:26:
I'm going to use it for a Pentium MMX system for Windows 95 and 3.x games, as well as DOS games all the way down to 386. I'm also planning to get a MiSTer, so a 15KHz monitor would also be helpful for the systems that were extracted to be used on a 15KHz monitor, rather than a TV.
 
Unless you want to use original lightguns with SNAC on the MiSTer you don't need a 15KHz monitor.
Just edit the mister.ini and enable the scan doubler. That will let any 15KHz console work directly on a VGA monitor with no other scaling occurring. There's no lag, but lightguns don't work due to the changed frequency. You can still use a modern lightgun solution like a Wiimote or RetroShooter just fine with any core that supports lightguns.
forced_scandoubler=1
If you go beyond consoles, you may need to use the real internal scaler which introduces a frame of lag. This is on many various arcade cores as those use all sorts of non standard refresh rates. But even the pros that compete in high level tournaments can't detect the difference in latency.
EDIT:
The One Demon wrote on Yesterday, 22:44:
Alright, thanks, I didn't realise they were using 31KHz already, sorry my understanding of this era ain't great as I grew up with consoles and we didn't have a computer until like 2007 or later 🤣 (Sega master system, SNES, N64  PS1, PS2, etc). I'll probably skip the 1084S monitor than, but I've come across a NEC Powermate/Multisync II that looks like a good fit for what I was originally looking for (I think it just came packaged with a Powermate system, not sure if there are any differences beyond the label on the front)
 
Ignore the above if you get that monitor. It supports 15KHz and can do 24op and 480i/p natively. So real lightguns might actually work on it, that's as long as the monitor itself doesn't do any extra processing of the signal.