Not all upgrades are available in each class, with the top of the line upgrades being reserved for the GT class. With enough cash you can eventually put together the monstrous GTS-R racing machine. Putting together the upgrades brings with it a bit of strategy as well. Sink all your cash into engine upgrades and you'll likely end up with a car that's too powerful to control. The tracks that you'll be racing on in a given class might also help determine what upgrades you purchase first.
Instant action is just that.
Buckle up and jump into a race. Your choice of tracks, number of opponents, number of laps and so on. Multiplayer is, yep, you guessed it, racing against other humans. Thankfully you've got all the connection options covered here. With four of us racing all on 33.6 or 56k connections it was pretty ugly.