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I ended up buying the Micro SD since I found it at a discounted price.
To address the issue wit the card sticking out, I bought a BaseQi Adapter made to sit flush with the 13" macbook pro. I think it fits a lot more stealthily than the Transend Drive too.

Then definitely get the 2015 Retina Pro, even if it's just the entry level machine. You'll be looking at the screen all the time and that alone makes it worth it. Performance is a nice bonus too.

You're the only one who can estimate how much storage you're going to need. For office stuff and your personal documents it's enough. Maybe you'll run into problems once your photos library or music in iTunes gets really big. But there are a few options.

Transcend makes JetDrive Lite: https://transcend-info.com/apple/jetdrivelite/
Up to 256 GB and you pop it into the SD card slot.
They also have a replacement SSD, but it's not available for the 2015 model yet:
https://transcend-info.com/apple/jetdrive/

Or this adapter: https://www.amazon.de/dp/B00QGLATK8/ (German link). It's a clone of the Nifty Mini Drive. A small adapter for the SD card slot that takes a microSD card. I used this and moved my iTunes library to it.

I also got a WD WiFi drive (My Passport Wireless). 2 GB of my data available wireless wherever I am.