Autopilot limitations normally specifiy a minimum connect height, it's 200 feet on the 767. I normally don't engage it until I've cleaned up the aircraft, about 3000 feet or so. On a good flying day I'll hand fly up to the cruise. Dealer's choice.
On aircraft equipped with a TO
/GA button (not the 767) it is engaged after stabilising the engines. Engine manufactureres recommend a stable setting, ie 70% N1 on a CF6, 1.1 EPR on a JT9/RB211. The thrust levers are advanced manually to this setting, stable settings are observed, then TO
/GA or it's equivalent is selected.
80kts is where many Boeing types autothrottle clutches disengage to allow the thrust levers to be closed in a rejected takeoff, if takeoff power is not set by this point it must be done manually.
Hope that's a start for you!