I don't care how great a game is... having only 1 is pretty tough. Being able to switch back and forth makes sense, unless space is an issue.
I'd go with a System 11 and a 90's DMD.
For System 11, I'd argue for something like Pinbot, Taxi, or SOF. I'd really wanna say WW here, but I doubt you could find one working for $2500 or less. Ideally you could get this pin for around $2k.
For 90's DMD, I'm partial to something like Judge Dredd or Shadow, both are findable at ~$3k. Road Show as well I'd say.
Shadow + Pinbot, I see them sell for roughly $3200 + $1800... right at $5k.
Judge Dredd + Taxi. I tend to see this combo more of the $2900 + $2100... again right at $5k.
Something like that. YMMV.