Cheaper then a NAS, a little under $40. It has an emulator on it. Can you change the emulator? To one that supports more systems? Or is to under powered for anything else?
You could make your own, and put the retro distro on it. They sell screens for SBC.
The ODROID-C1+ might work, with the ameriDroid VU7 Tablet Kit (3D Printed). You need the screen too. Can you power it by the micro USB port? Use a USB battery, I have one that I don’t use.