I've nearly settled on how I'm designing my new system... you you almost hit the solution on the head!
I currently use a 22" HP All-In-One touchscreen computer, but it's not rack-mounted. It sits on the table next to my rack which contains all the rest of my DJ equipment.
My backup is a laptop that runs my lights, but can also do music if needed. (The 3rd screen on the right on the speaker pole is for karaoke.)
In the past I used a rack-mounted computer with a separate touch screen, keyboard and mouse.
There are strengths to the rack-mounted computer, like it can always be upgraded...
The all-in-one is a nice package, but is more limited in "upgradability", as is a laptop.
In fact, the typical all-in-one is really a laptop motherboard inside a case with a large LCD screen, but no battery.
The one thing that helped me decide what I'm going to do is the UPS - Battery Backup.
To protect any kind of a computer other than a laptop, a UPS is necessary to protect it from glitches, brownouts, blackouts, etc.
With the All-in-one and rack-mounted systems, I've had to carry a 1U rack-mounted UPS all the time - that thing is heavy!
(Speaking of heavy, the 4U rackmount computer case I have is made of steel - that thing is heavy too!!!)My solution:
"Rack Mount" a laptop - no UPS required. Use an external Touchscreen LCD, Keyboard and Mouse on top of the rack. Additional hard drive storage, audio and video via USB connections as needed.