Requirements for AR Kit dev laptop?

Any of the new macs will do, just be sure to max the physical ram & buy twice as much HD/SSD as you think you'll need. ARKit is an API, part of the SDK that is included with Xcode 9. MBs have small screens, tho, so your work could benefit from an external LCD, or even a mac mini, at that point. Check refurbs in the Apple Store online.

You'll need a compatible iPhone 6~7 & iOS 11 to take full advantage of support for ARKit testing, etc. tho.