Just one point to keep in mind of considering older Macs.
There have been many references to upgrading both RAM and hard drive, but remember that not all Macs are upgradable.
I have a Mid 12 MacBook Air, 4gb RAM and 128 GB SSD, to the best of my knowledge neither RAM or SSD are upgradable.
Not a problem to me it works just fine, also I'm still using OS (10.7) Lion operating system, although the upgrade to 10.10 is free I have poor experience of OS upgrades with Mac and zero confidence in Apples willingness to support me, not withstanding that I am very happy with my mid 12 'Air.
After my experience with the 'Air I wouldn't consider any computer without an SSD, it makes an enormous difference to the feel of the machine. Also look for USB3 for speedy access to external drives.