One of the things I do with my Macbook Pro is use an external SSD to move pictures onto and off of my laptop, as I'm editing. If you have (or are thinking about getting) some sort of network attached storage, you might not even have to worry about buying an external drive for the iMac. Even if you decide you do need more disk space, I would suggest waiting until closer to time of that "need" than "just grabbing one because some person told you you needed one." Prices tend to go down and capacities tend to go up. So, *generally speaking*, the longer you wait, the more you can get for the money.