You want to buy a Chromebook that has the longest timeframe for Auto Updates. The main reason for this is that, along with phones, you want to make sure that the device has the latest security updates. The bad guys are always finding new exploits.
Google's Auto Update policy is that they will provide updates for 6.5 years from the manufacturer's original release date for that device.
List of Supported devices with their Auto Update Expiration date
Google Approved Chrome Devices