I've used 13 and 15 inch MB's for quite some time. The 13 inch is quite usable and you can always add an inexpensive monitor for use at home or work. I currently use a 15 inch but that was driven by the processor choices, not the screen. I do prefer the larger screen, however, so take some documents on a flash drive to BB and compare them on a 13 and 15 screen to see what you prefer. I do like the retina display over the non-retina display, having used both in 15 inch MBP.
Upgradeability is overrated, IMHO. Unless you plan to do something that needs a lot or RAM or disk space you'll probably never upgrade the machine anyway. The downside of a slower processor will be more limiting in the long run as it will be the speed bottleneck as time passes. Get as much RAM as yo can afford but don't worry if it's only 8GB. Quite frankly, if you wind up doing high end video you'll be wanting a Mac Pro anyway.
As for disk space, look at how much space your documents. videos, music currently uses. That will give you an idea of how much you'll need; after you decide what files yo can live without. I know people who insist on storing 100+GB of photos on an 128GB Air and complain the disk space is to little when the real problem is they haven't looked at what photos haven't been opened in months and offload them to an HD. If you do wind up doing large video file editing you can always ad a Thunderbolt HD which will be faster than USB3.0.
In the end, decide how much you want to spend, whether you want a 13 or 15 inch screen and buy the highest spec machine you can find at that price. Apple reforms are good deals, and BB's education discount helps as does the USPS moving coupon you can get at any post office, just ask for moving package. You could even ask BB if they'll match a refurb price with a new machine just to see if they will. In addition, look at B&H Video's (
http://www.bhphotovideo.com ) prices. They are very competitive and do not charge sales tax outside of NYC. I've bought camera and computer gear over the years from them and they are very reputable. They also have the best prices on AppleCare although the edu price is close.