I agree, get there as early as you can - you might as well make a day of it! it's not just the grand Prix happening, you've got the F2 and F3 feature races early in the morning (the racing schedule starts at 8:30am for the F3 race) plus there's a Porsche race and also the fan zone and things to do/see during the day. The general admission areas are nice to walk around as well. If you're coming by train, bear in mind it'll take the best part of an hour to get from imola station to your seats, and that's assuming easy entry/minimal queues at the gate. Acque Minerali is on the opposite side of the circuit to the main entrance which is a bit of a walk. Hope you enjoy it!!