(Deleted - just realised I was replying to something from two years ago!).

For the most recent poster - talking from very recent experience (last week) you don't really need to allow more than one hour from plane to train. Regular train tickets (as opposed to super-saver ones) are valid on any train, so it doesn't actually matter if you miss the train you were planning to take as you can always take the next one. I would go to www.bahn.de to book - it's very straightforward once you've registered, and you can print your own ticket.