Carsten -
Just curious, have you asked the Kirkham's if you can have your cake and eat it too? Meaning, can you order the original style steel suspension yet also have the upgraded modern brakes fitted to the original style suspension? I don't have a Kirkham (I wish I did)
but in the small bit that I have dealt with them on parts for my car, they have always been EXTREMELY accommodating, and always seem to welcome the challenge of a one off solution. Talk with them, there is likely something that can be worked out for you.
Also, I am not one who has all the history and knowledge of the original cars at my fingertips, but I can tell you through what I have heard that each and every item that I have heard about the original Girling brakes has been bad. I have never heard a good thing about them. For whatever that's worth......
Darn, I wish I was ordering a Kirkham for myself......good luck!
By the way, where are you writing from? Australia? Europe?