Steve Gillies at West London will be able to guide you through the ideal set up required-(though be wary, he may try and persuade you to become a fellow spring gun fanatic). Yopu will find plenty of good used scopes in the sales section-£150.00 should cover a decent scope and mounts
The fun in comp shooting, is learning to get on top of your kit-joining a club will provide invaluable help in the early stages of this learning curve.
Check out the HFT learners days that John Dove is going to be running at Lea Valley (details on their website)-perfect for new shooters