So long as you get a MacBook that runs Sierra you'll be able to run Xcode for some time. Regarding dev devices I'd personally recommend getting the 6S at least

So long as you get a MacBook that runs Sierra you'll be able to run Xcode for some time. Regarding dev devices I'd personally recommend getting the 6S at least so you can use all the latest APIs (specifically 3D Touch being the one you'll find in the 6S vs the 6) but if you don't need that you might be ok with something older.