^ Let's do this like maths:
White PCE (original model) + SCD =
CoreGrafx I (blue logo) + SCD =
CoreGrafx II (orange logo) + SCD =
DUO
The SCD (and the DUO) has a System Card 3.0 built-in, so you can play 98% of the PCE library with PCE + SCD.
If you want to play the ARCADE CD-ROM2 titles, you'll need an Arcade System Card (two versions exist, either one will work).
One game, due to quirks, will only play with System Card 1.0: Altered Beast CD-ROM. You can play with other system cards, but the game will crash at some point (I'll allow other folks to provide the details).