172 Cup at 18?! Bloody hell, I wouldnt have had the balls then even if I could afford it. Despite thinking I was relatively safe (and Im still on 0 points and 0 own fault crashes, touch wood) I am at least 10 times safer as a driver now than I was then (23 now), and believe me I am as sensible as they come.
5 years in a 1.2 has taught me a lot and I dont regret it for a second, even now I expect to drive the Cup for a year before I really start pushing it properly as although its a Clio the power available really is immense. The fact that a guy locally put his 172 through a hedge and into a swimming pool (passenger died) is always in my memory when I sense the red mist decending.
The moral of my story, even if you can afford it it doesnt mean its a good idea...but good luck!