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ingo
        
In an ideal world, what we need to know is whether (a) you can buy the car already converted (as appears to be the case with the Merc-based Hamann Hawk ^, plus others like AMG, Alpina, Crayford etc) or (b) whether you have to buy the normal car and then take it to the converters yourself. 
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On the Hamann-site
http://www.hamann-motorsport.com/en/special/inside-hamann/history/
it's not clearly to see, what is usual there.
As it seems, the offer all possibilities.
You can buy ready cars: http://www.hamann-motorsport.com/en/special/cars4sale/
but also just parts and surely you can bring your own car there with special orders for converting aftersales market.
Usually all these companies offers all options. Also at AMG and Brabus you can bring your car, you don't have to buy already done cars.