Autoeurope is a third party consolidator so the actual car you get could be from any of the common rental agencies there. The advantage is that autoeurope is set up to deal with international visitors who may not be comfortable dealing directly with a foreign agency.
(edit) I see I gave similar answer as below to your previous post. Since Rick Steves doesn't cover Poland well, we found the Lonely Planet Guide to be very helpful.
Places to visit? Any particular interests? Day trips or touring? Smaller towns worth a visit - Kazimierz Dolny, Sandomierz. There's the shrine at Czestohowa which has great historical and cultural significance to Poles. The nature preserve at Bielowiesza (home of the last wild European bison). Of course there are the concentration camp memorials and then the mountain resorts in the south, Malbork castle, the Masurian lakes, etc. Krakow is worth a few days if you want to go that far.