I'd be okay with either one really, though I'd rather just have the CD-ROM2. I just don't need another PC Engine with it.
If you get a CD-ROM2 unit, you're going to need an IFU. However, you could get a
Super CD-ROM2 add-on, but I think that's more expensive (and looks way lamer!
)
EDIT: You're also going to need a System Card, probably 3.0 or Arcade Card Pro depending on your preferences. I have a 3.0 I'm considering selling in favor of the Arcade Card, so PM me if you end up needing it.