Renting a car for Europe has generally been an uncomplicated experience for me. I most often use AutoEurope, which is designed to find you the best deal from the large rental companies. To date, I have been pleased with their prices. I always book well in advance of the trip, and look for the smallest car that can hold our luggage. Larger vehicles are harder to handle on narrow, winding roads, and in the congested cities. Parking is often a problem in the smaller towns. Gasoline in Europe is much more expensive in the United States.
You will pay much more for an automatic transmission vehicle, so get a manual (stick) transmission if you can. If you do rent an automatic, you must book far in advance as the availability is limited.
Never skimp on the auto insurance. It is expensive ($10-$30/day), but worth it. CDW (collision damage waiver) with a zero deductible or fault free insurance is recommended. On a road trip of any duration, your vehicle will be at least scratched, nicked or dented. A lot of my scratches have occurred while I was parked, and you will be probably be parking in rather tight spots. I like to enjoy the trip without worry
Another important fee that you need to consider is the drop off fee. It can be considerable if you are traveling through a number of countries and not returning to the country where you rented the vehicle. Make sure you get those costs before you finalize the reservation.
Another must for me is a GPS. These can be rented from the vehicle provider for a daily fee, but I prefer to bring my own from home. If you purchase a GPS in the USA, you probably will also have to purchase an insertable chip for Europe maps.
I went years without an international driver licenses (which can be purchased through AAA), until I was stopped in Switzerland (perhaps the last country in Europe that you want to be stopped in, though all the police do speak English) and was told to get one. I have never been asked for an IDL when picking up a vehicle.
More detailed information on renting cars in Europe is available at other internet sites. Rick Steves’s website has a comprehensive article on the many options available for car rental.