Respectfully, people say all kinds of things on the topic of registering s1's and the like that are not correct. So, don't be dissuaded. Just talk to the folks that have done it like the guy who recently registered a Carlton (very jealous of him.)
In this case (the s1) you import car parts (leave the engine out) or save the money, and buy an s1 elise (they are not expensive and there are kits to convert to LHD) have the POS Rover engine removed prior to shipping ( Plans Motor Sports is highly recommended to do this). Next, buy the Exige body kit from Elise Parts (lighter than stock s1 exige bodywork) and put all this in a 20' container ($2,500-3,500) and ship the car parts. There are a ton of DIY conversion kits. Register it as a "specialty constructed vehicle" (states do differ but most have a simple / similar process involving a police inspection, emissions etc ) and you have LEGALLY registered your car.
Go w the Honda as it is lighter than the Audi lump and a 2.4 monster is very easy to build. I have driven an s1 Exige on the road since 2006 (minus a three year upgrade). Mine has a little less hp, 485 but is probably 200 pounds lighter so pretty close on performance and the only thing I have experienced that delivers the same rush is an RSV4. It was not difficult to lighten the car to 1,550 pounds and soon, it will be in the high 1,400's. My S2 bared no resemblance to this car. I strongly encourage anyone that is thinking about this to DO IT! TTS (also out of the UK) has a kit that features the large Rotrex unit for the K20. Do a 24 with cams, a charge cooler, etc and you will have 600 hp (if you lighten the car a bit, that equates to almost 800 hp per ton). Then, move to Malibu and chase that black Venom around in a car that was $900,000 cheeper.