Friends of mine rented a car with a big trunk and shoved in their bikes (and hardshell bike cases).

Minivans are indeed expensive and unusual in Europe (smart people), so don't count on that. But many cars are big enough and you can move the seats around. Plan to bring a tarp/sheet to protect the fabrics from grease stains.

Visit Fiat or Opel's European web sites and try to figure out a car that would about fit your needs. Then you call the rental line and tell them you'd like a car in the likes of _____ because you have lots of bulky luggage. They will be able to find an equivalent for you...

Good luck!!