We rented thru Mileage Plus a Sixt car. It includes collision damage waiver. Do we need more insurance?
No. The car rental automatically includes legally-required second-party liability insurance. The CDW you have will cover damage to your car, probably with a significant deductible. If you buy "super CDW" to reduce this deductible, you're paying a pretty high percentage to insure that stated value. See also http://www.ricksteves.com/travel-tips/transportation/car-rental-cdw.
You do need an international drivers permit, obtainable at AAA for $20.
Did you try Europcar? I have had excellent service from this company on several occasions.
CDW...get a credit card that will cover this charge for you. I keep a card with a low foreign transaction fee from Capital One that also covers the charge for CDW. And years ago I had to use the CDW insurance. It was in the Czech Republic before many spoke English there so I submitted the car repair bill and wrote a description of how the window got broken...They paid no questions asked.
CDW can be more expensive than the car rental. Go ahead ask your credit card providers if one of them covers CDW while out of the country. Some will do it for 16 continuous days others will do up to 30 days in a row. Next see how much a competitor or the same organization will charge you without the CDW. And then see if you can cancel your current reservation. Be careful not to modify your reservation as the card offering CDW must be the one used to make the reservation.
This feature reduces my travel costs by hundreds of dollars per year.