Here is some info I've collected for some alternative hardware:
Base Computer:
Odroid C1: $35
USB Power Cable (5 Volts 2 Amps): $2
Plus a screen:
C1 3.2inch TFT+Touchscreen Shield (320x240): $25
Or
Odroid-VU (9-inch 1280x800 touch screen with speakers): $120
You could use virtual buttons on the touch screen. The VU is larger and has a higher resolution which would allow more room for the virtual buttons on the touch screen. It also has speakers for audio.
If you want physical buttons or external speakers:
C Tinkering Kit (has buttons): $16
USB Audio Adapter: $4
Misc. other stuff needed at some point:
Cat5/6 cable for internet (unless you want WiFi)
MicroHDMI->HDMI cable/adapter
Keyboard/Mouse for initial set up
MicroSD or eMMC for storage: ~$5-$20 from NewEgg or similar, depending on how much storage is necessary.
I'd be happy to build it. I don't have the budget to buy the parts, but I did buy a C1 earlier this year and didn't really know what I was getting into, so I got a few extras "just in case" that I realize now I don't need that could slightly reduce the cost of this project.
IMO a touch screen would be sufficient. No need for physical buttons and extra wires/kit required for that. The 3.2 inch screen would probably be sufficient, but then you'd need external speakers (and an audio adapter). The VU screen would allow plenty of room for Cartridge art (or something) and make it easy to scroll through a (large) list of carts with as much relevant info as needed.
It would run Linux (Ubuntu) so pretty much anything you can code there you can code with this stuff.
In fact, you could probably just make a Linux VM and code it up and have me test it on my C1 (allowing the buttons to be either touched or clicked with the mouse) and once it's ready and confirmed working we can order the parts and build the machine it will run on. Your VM would either need to be of an ARM device, or you'd need to somehow have your VM compile it to be run on my C1 (I know this is possible, but can't recall what it's called).