I still need to play Kingdom Hearts 1.5 and 2.5 on my ps3 but I know that they'll be extremely time consuming. Also I'm hoping for a "Complete Collection" on ps4. Should I wait or should I play them as soon as possible?
I say go for it. The games in those collections are the final versions in terms of content and graphical improvements. I suspect little of value would be gained in a ps4 release. 1.5 and 2.5 are also pretty affordable right now, a brand new ps4 collection might even prove more costly.