Question for you aficionados... which of the supported systems would be best for this? I was reading up about the different PCE it supports and it seems some have different revisions of the CPU. Would this at all affect how well they run with this addon?
Only the SuperGrafx and early CoreGrafx I systems have the revised sound chip, which fixed a popping sound that'd occur whenever there was a big change in volume levels or a channel turned on/off. Other than the sound fix, there's no reason this add-on would work any differently with 'em, so it's up to you to decide if the fix is worth worrying about.
Would I need any sort of existing mod in place?
Only if you want a region mod for physical hueys or video mods (component/svideo/hdmi).
How about color? I see this shell might be black in color. Would I be better off getting the CoreGFX II to match it ?
The CoreGrafx II is the light grey and orange one, so it wouldn't be a great match; the CoreGrafx I is dark grey and blue, so it wouldn't match but would probably coordinate better; the closest but probably not identical match would be a TurboGrafx-16, though the contrast with an OG (white) PCE could look good too.
Which of the supported is the cheapest/most reliable?
The TG-16 and three core PCEs are all fairly reliable. The only systems that really have issues are those with lasers.