The train will probably have all German cars or all Austrian cars. However, it doesn't matter where you buy the ticket: if you have a 2nd class ticket, you can sit in *any* 2nd class car. Even if you make a seat reservation, the reservation system will book you into any available car, whether it is German or Austrian. And if you have a reserved seat in one car, but you find another car which looks better to you and has some unreserved seats available, feel free to sit there!

If you can go 1st class for only 10 Euros more, I would probably do that. I usually travel 2nd class, and those cars and seats are just fine, but you do get more space in 1st class. In a 1st class compartment, you still have six seats - 3 and 3 facing each other - but the amount of legroom in between is much greater. If you go for the open-plan coach, you will find 2 seats on either side of the aisle in 2nd class, whereas in 1st class you have a single seat on one side and 2 seats on the other side.

Have fun!