Nothing official, but the V8 is nearly a forgone conclusion. The original M3 had a four cylinder engine that was basically an M1/M5/M6 straight six with two cylinders chopped off the end and the oil passages blocked off.
The next M3 will most likely have a M5/M6 motor, again with two cylinders chopped off. That would make it an oversquare high-rev 4.0 liter 32 valve V8 with individual throttlebodies per cylinder. Power should be in the 400 range, and I suspect BMW will try to "one up" the Audi RS4 with 420ish. Since the crank will be shorter than the V10, they may try to take advantage of the decreased mass and increase the redline, which will squeeze out some extra ponies. Or they may be happy with 408ish.
Based on the 335i Coupe's published weight of ~3500 lbs, I would guesstimate the M3 will weigh 3,640.... About 200 lbs. more than the current (E46)M3, which incidently is 200 lbs. more than the (E36)M3 it replaced.
I'm guessing the base price will be $55,000 USD. How did I get this number?
In the current M lineup, there is a roughly $10,0000 USD split between an M3 Coupe and 330Ci (25% premium). The same goes for the Z4 3.0si and M Coupe.
There is a $20,000 split between the 550i and M5 (38%); and a whopping $28,000 split between the 650i and M6 (42%).
Now, the best guess on the base price of the new 335i Coupe is $41,000 in the USA. But the difference between the new M3 and 335i is going to be greater than old M3 vs. 330Ci. Given BMW's recent trend towards moving up segment, I would guess the M3 will be 35% more expensive than the 335i. That gives you $55k.
If you're wondering why I'm dealing with US pricing instead of European pricing, I will point out that the USA is BMW M's largest market. The rest of the world will take a cue from US pricing.
0-60 in 4.4 seconds. 1320 in 12.7 @ 113-114 mph. Again, this is just me guessing. It should be a fairly quick car, but of course the real draw will not be just raw speed --12 second cars are a dime a dozen these days-- but a chassis that is gifted enough to make use of those 400 horses and refinement enough to make it a decent daily driver.
M