Originally I was planning on using a 'standard' arcade monitor, essentially a TV with RGB inputs, and a metal frame making it easy to mount. However, this all quickly changed when I was browsing Ebay looking for parts: I was able to get a Wells Gardner 19" XGA monitor. Unlike standard Arcade monitors, this is essentially a PC monitor in the metal frame, meaning no special hardware or software trickery to get it working. The main reason I want with this monitor, however, was the price, $135, The cheapest I had found for Standard monitors was about $200 so I was sold.
Hooking up the monitor was a snap, and the picture was great. However I eventually ditched AdvanceMAME in favor of normal MAME, which has Tweaked Modes that work just fine with this monitor, and look great, it's really difficult to notice that you're playing MAME on an SVGA monitor, and not just a really sharp TV.
I'd say the biggest problem we had with this monitor was mounting it. The original cocktail monitors had mount points along the border of the screen, but this monitor was only designed to be mounted horizontally, facing forward, not upwards. The huge weight of the monitor kept us from simply mounting it normally and pointing it upwards, as the weight would eventually bend the frame.
First, we had to sandwitch several pieces of wood so that it would be at the right level, and attach tabs so that the cabinet itself would help support the weight. One we had all these issues taken care of (we thought) we noticed a horrible thing... the cabinet wouldn't close up properly? What the hell? As it turns out the monitor's frame did not form a right angle to the base so when we mounted the monitor, it forced the top of the cabinet not no longer form a right angle as well. So we had to drill out the rivets in the monitor frame, straighten it, and put new bolts in place. For something that was bought complete and ready to install, this sure took a long time!