I believe that if the game is rare, buy it and wait until a better example becomes available.  Then you can upgrade. In time, you might find repro parts become available. I'd rather HAVE that title stored away as a "worst case scenario.

I've done this with rare games such as GTB Spirit, Bally Fathom, Xenon, Stern Flight2000 and Freefall.

If a SUPER rare game appeared - such as Iron Maiden, Viper, Voltan Escapes Cosmic Doom or Williams Solar Fire - I'd just buy it and "hope" !