For the most part, 1.7 should be fine. Earlier versions (pre 1.5) handled resources a bit differently and so the structure may be different.
Alan Storm provides some excellent resources for extension development as well as using Magento's framework in general. This might be a good place to start.
In addition, I find looking at the core modules extremely edifying. It will give you a good sense of code style and structure - you can start with a simpler one like Mage/Contacts or Mage/Review to get started.
Magento offers a good course called "Fundamentals of Magento Development" - for a while, these videos were free, but that is no longer the case (they also have a in-person course for $3850). The online version is a couple hundred bucks.