buy one that is cheapest to run and insure
get a couple of years No Claims under your belt then get a 100HP
will scare the brown stuff out of you but will be Sooo much fun
I'm 20 and my insurance renewal on my 100HP is coming up in the next few weeks. I can get fully comp cover from reputable insurers for between £450 and £500. I know that's expensive compared to what "old" people could get on the same car, but it's about the same as my dad pays to insure his Audi and he's in his early 50s with a clean history.
My point is, even the 100HP is relatively cheap to insure, as long as you've got a clean driving history. If I was 28 with a clean history, I wouldn't consider not getting a 100HP on insurance grounds.