If I buy the original Rocksmith in the Steam sale, do I also have to buy the disc import tool to be able to play the original Rocksmith songs in Rocksmith 2014 on Mac (as I had to on PS3) or will it just import the songs automatically?

Yup, you need the disc import tool.  It's required to transfer/update the licenses to RS2014.

Cheers.  I'll factor that into the price then.

You might want to look into G2Play (or any other Kinguin powered site?) for the base game since they're selling it now for like $15. Not that much of a difference with the current Steam sale, but buys a beer.