I am also just using a 2012 MacBook Pro Retina display system. Could not be happier.
The current system is a 2.6 GHz Intel core i7
Memory 16 GB 1600 MHz DDR3
It is configured with 512GB Flash Storage
In most cases I do not keep any images in the system.
I have images stored on external USB 3 hard drives.
To me Thunderbolt drives at this time are over priced and I have found out USB 3 drives works just fine?
One of my main external drives is the Seagate Backup Plus 3TB that I am getting Read/Write of about 174MBs average.
I purchased a Iogear USB 3 4-Port Hub
so I could have a number of USB 3 Drives attached at once.
No problem as they all mount and run with no issues.
Would defiantly go with the 16 GB for Ram as it is almost impossible to upgrade the ram with the Retina Display system.
Like Anders mentioned Macs are expensive but last. And they do rock.