For the difference between Overseas and Japan versions, go
here.
The only Ayumi release with 2 DVD's is currently the coming A BEST 2, and it is only going to be released in CD and CD+2 DVD. So don't worry about the CD + 2 DVD thing.
It is only up to you if you think the CD+DVD ones is worth it, you'll have to judge by looking at the price and comparing the contents. The contents of the CD is always the same in both CD and CD+DVD editions.
The easiest way to buy Ayumi media is if you own a credit card, like for example VISA, then I advice you to go to the previously mentioned pages
www.yesasia.com or
www.cdjapan.co.jp. Another good page is
www.hmv.co.jp, there's a button somewhere for translating the page to English.
For the record both CDJapan and YesAsia is exactly as trustworthy, the two companies are huge and there's no way they would scam you.
If there would be defections on any of your CDs when they arrive it should be easy to get a new one from both CDJapan and YesAsia. I actually think it's simpler if it's from YesAsia, they're the ones I've had the most experience with.