Your arithmetic is spot on, a 1 Tb external drive is a much cheaper option than a SSD upgrade. I still think that after 3 years as a primary device 128 Gb will be a bit small. For example I have 90 Gb of backups. These are such things as mobile device backups, icon backups, CS backup sync etc, etc, a total of 124 items. These mostly invisible files build up over time and the only real way to dispose of most (not all) of them is a matter of "Nuke and Pave" that is, a complete erase, clean OS install and restore from say a Time Machine Backup. Then I have 1302 cache documents ranging in size from 335 Mb to 12 Kb. So you can see that in three years a lot of memory is devoted to invisible process type files which are hard to find and dangerous to remove unless you know what you are doing.