Reddit mentions of AmeriGlo Cap Sight OR

I'll answer your questions then I'll give you my opinion/experience.

1. Whatever is more cost effective as long as it DOES NOT remove or MOVE your iron sight position.
   
2. RMR is the go to, but the Deltapoint (Leupold) is very nice.
   
3. Done very easily as most kydex holsters will fit (and designed to fit) an RDS)
   
4. low light = flashlights and lasers (think viridian, surefire or streamlight)
   
   My opinion on the matter:
   
   What kind of sights are you using? There are VERY high visibility sights available Trijicon HD or Ameriglo CAP, both of which I have used and would recommend to anyone with a G17. They are EXTREMELY high visibility sights.
   
   An RDS on a pistol has many pro's but the cons of them are HUGE. First off, the RMR is always on (non-battery/led), night time aiming is difficult and needs to be practiced a lot to get used to it. An RDS on a pistol will make the pistol nearly impossible to aim in the rain. The RDS gets wet you will have "bleed" of the dot because of the water on the glass. Have seen this in my personal experience and I am not a huge fan of RDS's partially because of this. Same line of thought, you hit your gun on something, breaking the glass, your gun is not difficult to aim with - you can always point shoot or try to slide-align (not accurate).
   
   If defensive situations are your real goal, then a red or green laser on your G17 will be hundreds of times more effective than an RDS. Also, low light, white light, may wash out the RDS entirely, depending on the RDS and the setting (TLR1 HL washing out the RMR, from what I have seen, same with some of the higher power surefires).
   
   Lastly, what is the price of getting all this done to your G17? RMR = $500 on a good day, slide work and/or new slide = $500, that's being liberal with the prices. Buy yourself a laser/light combo, get a good kydex holster for it and call it a day.