Fiesta would need a rear wheel drive gearbox and some sort of rear axle arrangement. Tom's Velocity (lovely kit!) is Sierra based so we just went to the scrapyard and got everything we needed. Since the engine was too big (for insurance) we got hold of an Escort 1100cc including 'box to put in.
I think the Avon is Sierra based isn't it? In which case get a Sierra and the engine seperately. OR just get the bits you need, but that will include the V5 registration document if yopu want to get an age related rather than Q plate.
Take a look at ALL the 'Locost' kits. I wasn't very impressed with the Avon. And visit
www.locostbuilders.co.uk