it's hard to tell how much exactly you need to pay to upgrade 2013 Fiesta to latest spec, as anyway during that long period of time you need to do engine rebuild, change some parts, etc, so then you rebuild it to newer spec, change parts for Joker ones, etc.
You can upgrade one of first Fiestas from 2013 to latest spec, Evo2 engine, all the Jokers, etc. Chassis is the same. Otherwise new homologation needed. I did some WRC rallies with one of the first cars with all the latest upgrades, so FIA scrutineering people think the same...

