You should be ale to find Erfurt at your German map. Bad Berka is about 20 kilometres east. It seems to be a village as small as Wacken city
And yes, of course you can bring coolers with beer and drinks and food as much as you like.
For "anything else you might pick up in Amsterdam" - here a quotation from the Festival ABC:
"Customs duty
The fact that Schleswig-Holstein is a little Bundesland , means that we belong to the category - great event - . That means that the customs duty are able to do controls all over the freeway. We can´t prevent that, sorry. So just keep an eye on what you take with you.
The Police will also do alcoholcontrols on departure of the W:O:A."