I would only consider Vodafone and T-Mobile as your options, these two have established cell networks, all the others borrow on these networks and as such tend to be at the bottom of the traffic prioritization.
From my experience O2 is absolutely awful for any 3G, they are building up their own network, but if your not in range of one of their cells you can forget it.
As for getting it, I'd wait till your here, you are mandated to provide your passport details to get any SIM card, so they probably can't service you overseas.
As for getting a 3G stick and SIM once your here, just rock on in with your passport and a bunch of cash and you can be setup that day... however as for data plans...
Germany is crazy expensive... so don't expect to do any downloading over it, but you should be able to browse the net and check email 'n stuff.