I really should have explained in the OP, basically I will be buying the car but as a kind of present to my dad who isn't very well he will be having it for the first 1-2 years. I will be driving around in his 1.2 Corsa

Then after I get the No claims up I will be taking the car off of him. I have found deals as cheap as 5k for a 350z, and I assume other cars that are less than 3.5L could be even cheaper
Even after 2 years of no claims, it'll still be ridiculous. You won't get a quote under about £3000 for one until you're over 21.
All of this is also dependent on you going for 2 years without making a claim don't forget.
Also, you're forgetting fuel consumption entirely. I have a 2009 KA, which is one of the cheapest cars to insure, and run, and even that costs £50 or so in fuel every 2 weeks - obviously depending on how much it's used...
There's absolutely no point in thinking of getting a cool car until you're at least 21 and have a decent job.
If you get something old - like from 2003 as you've suggested, you'll just end up having to pay hundreds within a year for repairs. It'll need a new parts like a cambelt and brake pads - not that they're that expensive, but it all adds up unnecessarily...
Get something newer - if you've got £5000 to spend, get something from 2009. They'll have been under warranty generally for the last 3 years and so had regular services, and then traded in once that has expired. Therefore it's more likely to be reliable.
Look at Fiestas, Corsas, Polos, Kas, Clios. Build up your no claims, sell it on after a year or so and change it - that way you won't lose as much in depreciation, and insurance will stay low.
For example, this year my Ka is £1100 to insure, next year after a year of no claims that goes down to £600. Absolutely no point in wasting money unnecessarily on insurance. Annoyingly - I was insured for 9 months previously, but it doesn't count for anything unless you have a year...
Tbh, I'd be scared on the roads if 17-20 year olds could afford to drive round in muscle cars...