Joe.
On behalf of the entire world I would like to politely ask you to stop making a constant stream of threads all on the same subject.
You recently made this thread -
http://www.audi-sport.net/vb/a3-s3-forum-8l-chassis/175570-a3-1-8t-agu-k03-modifications.html
Which has had some good views, many replies and a lot of information. If you have further AGU tuning questions please refer to your original thread, use the FAQ tuning thread in the top of the threads list and use the likes of eBay or classifieds to answer your questions on pricing.
300bhp isn't possible, reliable, or maintainable with a £2500 budget. And quite frankly, you're not ready to have an 8L modified to that standard, it will be more trouble that it is worth.
Glad to see you are getting your license back. Try and keep it this time; I would suggest a 300bhp A3 isn't the best option. Considering this forum is monitored and sponsored by insurance companies, you will need to declare your modifications. As somebody who has just got their license back I am certain this will cost you £2500 on its own, without budgeting the maintenance and running costs which come with a car running double its standard power.