Working on a 90's arcade and my source is HDMI. I want to use the NES zapper which i think runs at 15KHZ but regardless, I want that retro video look. Any idea what I need as a screen/monitor? How would I connect the HDMI to the screen? Recommendations?
Are you building/refurbishing a 90's arcade cabinet? What video game source is exclusively HDMI? (PC GPU's have alternate video out methods, as do gaming consoles).
As for the NES Zapper, it was a light gun that used an infrared beam bounced off of a black & white screen that would flash on the screen for a fraction of a second (15KHz may be right). White was a hit and black was a miss. Because the light gun AND the game are both coded to produce and receive that flash at a specific time, you won't be able to use the NES Zapper with anything other than a CRT TV (think non-flat screen TV's), although I may have read of someone using a projector once, but I'll chalk that up to internet talk.
As for that "retro video look," you're talking about scan lines, which again came from CRT TVs. Today, some indie retro/pixel games will build that option into the game and emulate the effect in the software. For games that don't offer a scan line option (it's a fairly uncommon feature), you can use SweetFX to forcibly add an after affect to your games via the advanced options (there should be one for scan lines).
In short, you can't use an NES Zapper with a flat screen TV. You can add scan lines to anything you like. If you want to solve both of your problems, use an old Cathode Ray Tube Television, which are pretty much free these days.