Try this place in Gdynia (10 kms from Sopot)
They say at the link (in Polish) that they have classic Daimlers to rent.
Because it is probably a Polish-only company you might want to get a trusted relative of your fiancee to help arrange it - a good way to bond with one of her brothers / male cousins?
Also for what its worth my wife was offended when I suggested we hire a classic jag for our wedding ("why do you want to get an uncomfortable old car! You dont want to hire a new one for me?!") ;)