Shortly after getting the car, I bought two sets of ST170 alloys off eBay, both for very reasonable prices.
For now, I've fitted the worst 4, which also had the least tread. Gonna use up that tread as much as possible then replace the tyres with winter ones, and they'll be my set to use over winter... not so bothered about salt ruining the paint/lacquer, or scraping a kerb hidden by snow etc.
The better set have plenty of tread left on the summer tyres, so hopefully won't need to replace them for 1-2 years maybe.
The breakers where I got one set had most of the ST170 left, so I went back another day and got the front hubs/brakes and complete rear axle for £150. Very happy with that as eBay prices seem to be £400+ for both of them.
I'm waiting for the rain to stop for a few days before I swap the brakes over... and hopefully in the meantime I'll get the braided hoses too.
Before, with Zetec alloys
After, ST170 alloys (no idea why the arch gap on the front is so big... wasn't once I turned the car round though)
After, ST170 alloys
After. Old & New. My car (dad's old one) and Dad's new Focus.