Also be careful about the rental seats. Type and quality varies greatly not only by company but by country. For instance. We rented a car seat in Florida from Avis and it was standard, molded car seat that you would buy for your own kids (I think it was a Cosco, but don't quote me). When we rented a car seat in England from Avis it was styrofoam with an elastic cover. You could easily break it with your hands.

Since then we always bring our car seat. There are several lightweight and portable models available. The Radian Car seat folds but is heavy (22 pounds), Compass Boosters are lightweight but only for kids over 30 pounds. Other people have been very happy with the Cosco Touriva.

As far as minivans with seats built in it will likely be a hit or miss thing. Even if the rental agency tells you they have one there is no guarantee it will actually be there when you show up. All they guarantee you is your car class, not specific amenities (except of course satellite radio, which they are happy to charge you for).