It depends upon what you want it to do. Suspension bolts? 1/2" drive, 500Nm or more and you will still need a breaker bar .Engine bay for ancillaries and maybe 3/8th . 1/4" drive is for light stuff only in my experience .
Milwaukee M18ONEFHIWF12-0 18v 1/2" will do everything your ever need other then crank pulley bolts maybe . I got the bigger brother Milwaukee M18ONEFHIWF34-502X FUEL but use a reducer most of the time. Main impact tool i use is Milwaukee M12FIW38-202B 12v as it`s strong but small and light so easy to use in the engine bay.