If I were going 650b, and I will be not, I'd be buying a full bike. It's just a PITA.
New wheels, new tyres, new fork, new spare tubes. It all adds up. Frame only means that you'll need those things, so it quickly becomes expensive due to the lack of spares about in the market.
Personally, I'd wait another 6 months and see if there are some of this years' frames discounted. CRC usually have a blow out deal from Commencal frames in October -> December, so the Meta AM 650b will possibly be discounted then.
Parts wise, as an idea you can reuse...
Shifters, Rear Mech, Chainset, Cassette, Bars, Stem, Brakes, Saddle, grips, pedals.
What you might not be able to reuse: Chain (as the chainstays may be too long), Seatpost, Seatclamp, BB (external or pressfit), Fork, Wheels, Tyres.