I hope you have gone to bigger brakes up front. If you haven't, then the rears don't need upgrading. I had the Wilwood 6 pots up front and to upgrade the rears, I did the SVT brackets and larger rotors.
To do the cheapest rear upgrade, get a set of 2002-2004 Ford Focus SVT rear caliper brackets. When I did mine the brackets were somewhat available, but it seems as though they are more difficult to fine by themselves. Getting the calipers with the brackets attached are probably your best alternative. You will have to eat the core charge, but you can always sell the calipers on Ebay, to help recoup your money. Then next you need to get a set of rotors for the 2002-2004 Ford Focus SVT and your rear upgrade is complete. If you haven't done so already, get stainless braided lines for all four corners.
The calipers with brackets is probably around $250.00 for a pair ( including core charge), throw in another $100.00 to $130 for a pair of rotors. Your total cost is around $350.00 or so, much cheaper than the Wilwood kit.
To make the Wilwood rears work, you need to change your rear hubs to the drum version. I'd assume the standard Fiesta has drums. So to sum it up, going with the Wilwood rears, you need to add their parking brake cable kit, add their braided line kit, buy a set of hubs. For all that you are in the neighborhood of $1500.00 bucks.
Since it appears that you are new to this forum, Whoosh Motorsports is kinda the place to go for aftermarket parts for your Fiesta. Ron (owner of Whoosh) is a very reputable online retailer on this fourm, quick communications and top notch customer service are his traits, you can't go wrong buying your parts from him. I've provided a link to the Wilwood rear kit from his website below. Good luck.
https://www.fiestastforum.com/threads/front-and-rear-brake-upgrades-installed.16801/
https://whoosh-motorsports.myshopif...-combo-brake-parking-brake-kit-2014-fiesta-st