Never heard of this, nor seen those vehicles. Apparently when the rear wheels are as close as they are (in the picture) the law sees it as 1 wheel. Why can't you wait 2 more years and then start driving is beyond me. Save out 5000 euro, and all the frustrations..
Cost to benefit:
The most modern cheapest brand new VW Polo has 75hp and weighs 955kg (empty) with a total weight of 1570kg (I also wonder if the rear axle can still bare 800kg?). It will set you back €12.750 (10752 GBP). The acceleration from 0 to 100 km/h is a whopping 14.3 seconds. Add the 'ellenator' cost of around €5200. And leave with a freakazoid.
I was reading about these the other day. Apparently it's to do with a strange German law that allows kids as young as 16 to drive 3 wheeled cars. They have to have 3 wheels, no more than 20hp and not go over 55mph! I'd sooner ride a bike :D