Congrats on getting you new Legacy! I love my Legacy, but I really wonder where you're going to put a big battery on the thing? There's really not much room in the cabin area, and the standard battery compartment all the way forward is rather small and fits between 2 non-movable bulkheads.
Unless you wanted a big battery box right where your feet are by the short 1/2 height transom and under the tiller, I doubt there is clearance for a big battery under the seats. There's a small diameter (about 6"?) waterproof screw-on hatch on the rear of each seat, but you will not be able to get the battery through it. I'm kinda doubting you could shove it down the quarterberth under the seats either from inside the cabin? Also, if you were to have open holes for wires leading to the battery under the seats, you run the risk of sinking the boat if you were to get water that came in over the transom in following seas. The cockpit itself is self bailing, but the area UNDER the cockpit is not.
Before buying all this stuff, I would recommend waiting until you have the boat, and taking a good look at the real clearances you may have to see if it would fit. I know it's nice being "green" with electrics, but my 4HP Tohatsu barely used 2 gal of gas for the whole season last year, and I must have used it a total of 25 days.
As for your question on what racing class the Legacy belongs in, I can guess the "slow and cheap" class? Only kidding--I'm not a racer. With a 17 foot boat, it's not fast, but 5 knots sure feels fast on a beam reach with the water so close!