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.

I thought they were selling the car, but indeed if they just make tuning of BMWs that people bring to them, it is not a make on its own.

I can replace the badge on my Kia with one from a Porsche, but that does not make it a Porsche. We need supporting evidence about what models Hamann actually made and sold or whether they just offered secondary conversions on E34s (and if wheels and bodykit here are Hamann). Or sold badges to anyone who asked nicely.

Why? It even has its badge replaced

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.

Depends if you consider the company as a make on itself.
On IMCDb we liste these as their own make, like Alpina: http://www.imcdb.org/vehicles_make-Alpina.html
For Hamann we only have one which is Mercedes-based: http://www.imcdb.org/vehicle_533188-Hamann-Hawk-C197-2011.html

Hello, people! In Kiev, found the BMW 5-Series E34 with Hamann logo. Do you know, how correctly in this case will be written the name? BMW 5-Series E34 Hamann or BMW-Hamann 5-Series E34? Or in some other way? Also interested in what year could be produced tuning?