Using a 780 is not a good idea if your processor is a piece of shit.
They're also around $650.
Putting my system together cost $3500, the liquid cooling setup added another $1000.
While liquid cooling isn't mandatory, it's pretty pointless to buy such high end hardware and not max them out while remaining cool.
Newegg saying combo units aren't necessarily 100% compatible is just to cover their own asses. It wouldn't make sense to sell it and completely screw you with mismatched parts.