7 am on a Sunday is most definitely NOT peak time. Sunday afternoon is (with people travelling home from week-end breaks), but I don't think you need to worry about overcrowded trains early on a Sunday morning.

You should be able to buy your ticket online and print it out yourself (that's what I did for an ICE journey from FRA last year). Make sure you buy a full-fare ticket (as opposed to a saver fare that is only valid on a particular train) - as others have said, full-fare tickets are valid on any train, so if you miss the one that you planned to catch you just hop on the next one and find an unoccupied seat - of which there should be plenty at that time on a Sunday morning.