WTF?
For a start, I assumed that the OP's car wasn't the charged version of the 1.4. I could be wrong of course. But even so, while turbo / injector failure is a possibility, it's by no means guaranteed.
And where does another minimum £1k of other running costs come from? Sure, maybe a set of tyres every year instead of every 2 years, but, assuming it's not running bling 19's or something, that is only an extra £200 or so. Basic servicing is easily done yourself, so another £60 on oil and filters should easily cover that. And these are costs that would have been needed every 2 years anyway likely, so the actual extra is only half that.
Brakes, yeah, they might need changed more often, depending on the actual route, and speed of driving. But that's only needing changed every 3-4 years.
I suspect his fuel plus £50-70 per month for servicing, tyres, emergency fund should pretty much cover it.
The thing is, he was only talking about £200 per month or so for another car anyway, so unless it's a new, small, cheap ecobox, that's about a £10k car on HP. So not a new "something nice", which would be likely to have some issues anyway being out of warranty. And sticking these sorts of miles on a new car kills its value.
Moreover, he has also stated it's only for around 6 months. So is only around 10,000 miles. On a 100k car, another 10,000 miles in 6 months isn't going to kill it. So long as it's in decent nick already, then there shouldn't really be any "extra" costs.
I would just crack on. Do the trial in the new year, see how you get on, if you feel you're ok to do it, then crack on with it. Fire £50-odd away every month "just in case", and don't sweat it.