The first question that would come to my mind is, how many miles do you do per year? If you're doing fairly average mileage, I'd be very cautious about putting loads of money into a toy if it compromises your every day car. No matter how awesome your weekend toy is, if you end up doing 10,000+ miles per year in some shitbox and only drive the toy a handful of times a year, what's the point? The every day car is where you spend most of your time, so I wouldn't sacrifice it too much for the sake of something you only drive on the odd occasion.

I'd finish the Fiesta first, then see how you feel once you've driven it a bit. You might drive it and hate it (plenty of people do when they drive old dream cars), or you might love it. If you love it, maybe see if you can drive a Cossie and see if you want to make the switch. A Cossie is a huge investment these days, as prices have obviously gone bonkers and they require plenty of looking after. That's not to say you shouldn't buy one if you can afford it, but there is nothing worse than sinking loads of money into something that you hardly get any enjoyment out of it, especially if you're sitting in some shonky base model eurobox the rest of the time.

I'd be tempted by a bit of a compromise. Nice, modern performance car for every day, then keep the Fiesta as a toy. Or for the cost of the Cossie, especially an Escort, you could find yourself in something very, VERY nice. For the c.£40k people are asking for Escorts, you're into Porsche 996 Turbo money. I know which I'd rather have.

Sooooooo true