Although PS3 alone would still suffice. Put in there KH1 with FM content, KH2 with FM content and maybe Re: CoM on top and I bet you'll get quite some folks to buy that stuff, especially in regards to preparation of the final chapter with KH3.
I won't necessarily count in Birth by Sleep since it's relatively new (and four games in one collection seems a bit much), but on the other hand considering BBS's overall importance to the series and it being also on a Sony console I won't rule out the possibility. To speed the people up to date with the rest they just could include an expanded Memoirs/Theater section within the collection which also covers Days, Re: Coded and Dream Drop Distance.
Like i.e. that when you finish playing KH1 you unlock the coverage of Days in Memoirs as well, when finishing Re: CoM you unlock Coded (due to the erased notebook) and finishing KH2 gives you additional coverage of Dream Drop Distance.
This would summarize the whole series up to now, give some additional gimmicks and would therefore be worth spending some bucks for.
That sounds like a great idea- I'd definitely buy it. Fingers crossed!