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. Many firms started as (b) and then eventually reached (a) - eg Koenig, RUF, Gemballa, Kahn - and sometimes going further through manufacturer recognition (Alpina) or even buy-outs (eg Mercedes AMG, Fiat with Abarth, Renault with Gordini and Alpine). There are also parallel scenarios with coachbuilders - eg Ford with Ghia and Vignale, Radford Minis etc. And to complicate things, many boyracers buy badges for their homemade unofficial pimping.

So we just have to take best decisions we can in each case. I know nothing useful about Hamann, but guess we've got the Merc correct as a self-standing model, but if that BMW ever appeared we should enter as normal BMW with Hamann in added info. So Hamann becomes used in 2 different ways. There is a third way where it becomes a defined model in the manufacturer's range eg Renault 8 Gordini, Mini Cooper etc.

Always going to be a messy area - we just have to cope as best we can, but staying flexible.