Yes, im pretty sure you can change it on a Fiesta.

Youll need to find a dash kit likely. Or have a fiberglass one made. It is possible but would be $$$. There are also line converters (instead of level/rca), some bash them but I have used them in customers cars as a cheap alternative to save people money and they work really well. I haven't ever touched a later fiesta nor have I seen any parts for a simple slide in headunit install.

yeah you will need [ Register or Signin to view external links. ] then get any single din radio u want

its cheaper here: [ Register or Signin to view external links. ]

If you order a headunit from Sonic they will give a you $30 credit towards your install accessories.
That includes the dash kit listed above, the wiring harness, and an antenna adapter for the radio.

It's good that they make a full size dash kit for your car, bad news is that it's one of the more expensive ones.

Depending on the headunit and if you choose to install yourself vs shop to do it. This could cost upwards of $400+ depends on which route you go.

A normal single din/double din dash kit wouldn't cost more than $20 online.

Just buy the all the items needed to install a system then take it somewhere to be installed, clearly you don't know much so if anything you might screw something up. Best bet is to buy a sub, amp, tweaters, maybe a horn and boom, you wont have ear drums anymore. But also you will need a head unit but you can always splice wires to get it to work with your current setup. but iwould let the professional decide what head unit you need so you dont have to run back and forth. also make sure the amp you buy supports the wattage of the tweeters, horn, and sub.