Given your budget, I would vote against a car for all of the trip but recommend the train for long distance trips.

I also recommend to skip Munich during the Oktoberfest, Budget and Oktoberfest is a contradiction in terms if you do not arrive in the morning by train and leave by train. Luggage storage might be worth to consider.

Regarding your hotels, maybe have a look at the cheaper accorhotels.com brands like Ibis Budget, Ibis as well. Overnight Parking in big cities are around 20 EUR (often more expensive, not always cheaper; usually to park a car is 1 - 2 EUR / per hour in the parts not right in the city center).