It depends on circumstances.
My first car was a Skoda Citigo Sport 1.0, when I was 17. I did it on PCP. It was 1000 down, 121 a month for 3 and a half years. I had a stable student job and so had no issues ensuring I could meet the payments. I never had any financial problems with it, and it got me onto the car ladder. For some people, that might not necessarily be possible, if your employment is (for example) a 0 hour contract, or hours fluctuate month on month, but for me it worked.
My girlfriend's first car was a £500 Kia Picanto. In her 3 years of ownership, she's probably paid 3 times the value of the car in parts and servicing costs. However, her hours change regularly, and so couldn't commit to a monthly payment. It's worked for her.
If you have the income to support £140pcm, plus the general costs (servicing, fuel, insurance and tax [ I know you're saying that your parents may cover the costs some of this ]), then it's certainly not financial suicide.
For what it's worth, successfully going through a PCP scheme, and making all repayments, is great for your credit rating too.
Incidentally, I am a car enthusiast, and it's so nice to see someone being sensible and pragmatic. Sure, I'd have loved my first car to be quick. But the 1.0, with some sporty body kit and stripes did the job at the time. You'll work your way up, and your goal by 25 is not unreasonable at all (although, worth remembering new things will come out in that time, so you might find something newer you prefer!)
Worth exploring the Ibiza deal too. You might find it works out cheaper, if they have a larger balloon payment at the end, despite the lower dealer contribution. PCP is literally a numbers game, and is worth exploring all options.