I travel overseas and so I find these global posts interesting. It is always a challenge to sort out the complexities of how to use electronics with wireless access in a global world without incurring extra fees. I live in the US and have a Kindle 3 Wifi only. I have interpreted the Amazon support information to be that if I travel to Europe, for example, that I will be able to download books via Wifi free of charge. I will also be able to download books (free of charge and without a wireless connection required) if I purchase them through the Amazon.com website and then transfer them via the USB cable to my Kindle.
Your post implies that you are interested in the Wi-fi only model. Therefore, it seems as if the cheapest cost is to buy with your US Amazon account and have it shipped to your US address. Also, if you buy a Kindle shipped to your US address, then it will come with an AC adaptor. The adaptor is rated 100-240V and 50-60Hz so you should just need a simple US to European plug to use an electrical outlet. I have provided some links for you below of where you can find help info at Amazon.com.
I suggest you read through this help page on the Amazon.com website which is for "Using Kindle if You Live Outside the United States".
http://www.amazon.com/gp/help/custom...deId=200505470
If you buy a Kindle shipped to your US address and register it to your US account, then I would hypothesize that means you will be treated similar to the "U.S. Customers Traveling with Kindle". There is some additional info on this help page.
http://www.amazon.com/gp/help/custom...deId=200505490
Also I suggest you look at the User's Guide on the Kindle Support page. Section 1.6 has some great information. You can find the User's Guide at this link.
http://www.amazon.com/gp/help/custom...deId=200317150
Best of luck, and I hope you enjoy your new purchase.