Ok so any recent Mac will work just fine for iPhone development, as long as it supports OS X Mavericks. You can get a Mac Mini even, although I did the same thing and ended up switching to it as my primary computer and ended up upgrading to an iMac anyway :D
You only need the $99/year iOS Developer membership once you're getting ready to deploy your app, or if you want to start developing apps with Swift (because you'll need the iOS 8 beta and the beta version of XCode).
1) When I first got started with iPhone development, I had about 3 years of dev experience. I think it took me about 3 weeks from knowing nothing to having an app ready for the App Store, although it was a fairly basic data-driven app that read from someone else's API. Depending on what you're making, you can speed things up a lot by using Parse or Firebase or another web-based data store as your backend.
2) Stack Overflow is a good resource, and there are a ton of communities and resources out there. Once you get going, you'll have a better idea of what to Google for. Apple's documentation is also excellent and has good beginner tutorials.
3) As you're a .Net developer, I recommend NOT taking the "easy" road with something like MonoTouch (aka Xamarin.iOS). It may seem like it's the path of least resistance, and it does have its fans, but I highly recommend just learning how to write native code. There are a lot more resources out there for you to use and learn from when you're writing your apps in Objective-C. In fact, even though Swift is the new hotness, I'd suggest making a simple UITableView based app with existing Objective-C based tutorials first. Then throw that all away and start learning with Swift. It will be a little rougher to find help since Swift is so new, but you'll have a better grasp on how to look up the Obj-C based solution to a problem and convert it to Swift in your head.
Good luck :)