Consider the color.
Consider whether it will be comfortable for her, not just you. See if you can get a woman her size to ride in the car to judge, especially if she's short or tall.
Be ready to cheerfully live with worst-case costs. If there's any possibility they could cause bitterness or regret, pass. Trying that as on couple purchase could work, but on an exotic gift for a special occasion, absolutely not.
Little things will make the gift more special. When you tell her you have a gift for her, give her some driving gloves and a pass to a performance driving training course. Tell her you've arranged for her to drive a Ferrari 550 (or whatever you choose).
Get a meaningful keychain (like a tiny copy of her Ph.D.), and have someone at the track have the keys, insurance, and registration on them so she can't look at them prematurely. Don't give her the helmet yet- it will obscure some of her reaction for the video, and if she swings around in excitement, she could hit you or the car.
Right when she's about to get in, hand her the keys, insurance card, and registration, and ask her to be careful with her new car. Have a bouquet and reservations to a nice restaurant ready when she gets out of the car. Make sure someone close to her (a sister would be good) is taking a video when you give her the pass, when she gets in, and when she gets out.
Congratulations on having a hard-working wife and plenty of money. I hope you appreciate them.