Some sculpts are very unlikely to change (or change very little) such as warriors or the monolith. Coincidently, warriors are necessary and monoliths are one of the best things necrons can field. I would say go ahead and go for it but avoid immortals, pariahs, tomb spyders, and flayed ones. I can see both the rules and sculpts (metal => plastic) changing for those when the new codex comes out. Perhaps not all but a couple definitely.
Their most effective stuff prolly won't get new models, besides said warriors and monoliths, the destroyers, and scarab swarms will probably stay the same and both are fantastic units. Only thing to be wary of is getting a c'tan as most rumors I've heard have them leaving the codex and going apocalypse. I would hold off on those.
In any case, you could make a fairly strong army (for crons anyway) using models that are unlikely to change.
Now as far as rules go...well that's an entirely different story. We can be pretty sure WBB and phase out will change in some way, probably the gauss rule too.
There are some rumors for ya from BoLS.
Oh and for the next six months if you want to win with them, play as brutally as possible.