It has been a long time since my service days in europe but I have a few suggested things to consider that might help.
Driving in europe was much more of a luxury than here in the states. Licensing, Insurance and Fuel cost were very costly.
I don't know how much it has changed but back in the 80's a german license was a couple of grand and they were not recognizing the International/universal licenses. I think that has changed somewhat but check out the licensing requirements.
Next was insurance,....which was and, from what I hear, still is pretty stiff. You could easily be looking at another grand or two depending upon the type of license you are able to use(International or some new euro union specific one vs your US one). Also, renting vs brnging your own or buying could be different. The insurance offered when renting could be what is the deciding factor for your question.
I hope this helps a little and hope more that you go sooner than later. The ride from Frankfurt heading south through Bavaria and through the mountians into Italy is a trip that should be on every riders bucket list.
When you go, swing through the Wiesbaden/Mainz area of Germany. Don't know if still the same but there once was a road up in the mountains behind Wiesbaden that is called the Whisper (wispertel?? ...its been 20+ years) It was a beautiful stretch of road, and worth the detour if its still undeveloped(Will try to find out and get back to you on this).
Later,
Freelancer