[jambo]

Weigh up what you want out of the bike, I'm no racer and the above replies are all from people who do actually race. However, if you're in it just for fun perhaps a custom seat subframe and plastics might not be worth it. I'd imagine you can always make changes through the season, so change the things that are holding you back as they become an issue. I'd imagine whatever budget you do have would be best thrown at tyres and suspension at first though changing the plastics will probably not cost you much once you've sold the stock ones off the bike.

Edit: to clarify the above the stock front fairing has headlights, mirrors, indicators and the plastics themselves, all of which sell for a fair bit, so an aftermarket fairing may well be worth it. Just don't get sucked into the thought that you have to change everything before you start!