Great news, The M4 makes the 335d look like a lazy slug performance wise you will be very happy.

What to watch out for...not much, there are few issues. Make sure the air con works properly. Check for bumper scuffs at the front its very low. Make sure the HUD is the coloured version, if its B&W its old and needs software updating. Make sure the M logos in the seats light up properly. Tyres are always an issue, Check it goes into reverse and out again properly if a DCT (if its slow its software again). DCT is fairly bullet proof. Like all BMW its very mileage sensitive and make sure the big services were done on schedule and oil changed properly. Bring a Carley and read the codes off the car.

Have you driven one already, its important to do so for comparison. The suspension should feel very tight and the steering heavy, there should be no vagueness at all. Ride is hard that ok but slackens off a lot on comfort.