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.
I do have an network attached storage on my macbook. I use BackBlaze and iCloud but I am still thinking of buying an external HD though. I've already waited long enough for a desktop. I've wanted/needed this new desktop for awhile now. My parents are going to buy my desktop for a present to me. Though I still want to make sure I get the right one before I let them know which one it is.
