Hi,
we did theDanube from Passau to Budapest six years ago - I can not really recommend it. There are too many cyclists, and the landscape is only partially beautiful. Many, many kilometros you are cycling on the side of a canal which is a litte bit boring. The beautiful parts are the "Schlögener Schlinge" (just after Passau) and the "Wachau" (Austria), which is really pittoresque. There are bed&breakfastes all along the way, getting a room was no problem. In Hungary cycling was difficult and dangerous, there are no cycling ways. Finding a room was no problem, the people are very nice and the food in Hungary is wonderful.
Did this help? Best Wishes, Alcina