If you just want the simplest way to provide backing tracks, and you don't need to send MIDI messages to any external devices, you could mix all your songs down to audio files and get a rack mount CD player (old school) or put them on inexpensive MP3 players.

Just send the output from the MP3 player to the PA. A $50 MP3 player will hold wayy more than enough songs for a gig... and they're cheap enough to carry a backup in case something goes wrong (not that anything would ever go wrong on a gig.... just sayin')

Another nice thing about MP3 players (besides price and convenience) is the built-in ability to queue up a set of songs or play one on the fly for a request