Best budget setup for home and on-the-go

Budget: $3,000 +/- a few hundred dollars.
Use: Editing/processing GoPro videos (4K), DSLR photography, moving large files between computers and Firewire 800 hard drives, note-taking, homework, paper drafting, research.

Looking for a system as a student to take on-the-go for note-taking, research, homework completion, some multi-media use, emails and client/work communication, but still can handle whatever I may throw at it within reason. But I also need a base PC/MAC for home use that can handle editing/processing GoPro video's, some DSLR photography and can still be usable with an adapter for FireWire 800 drives that have a lot of old university documents, GoPro files and my old photography backed up to.

I was considering a 21.5' 4K iMac and a base MacBook retina. It would be slightly above my budget, but I felt it could handle my needs better than my 2010 MacBook Pro could currently (Which the logic board, hard drive connector and airport utility tool are all failing).

 

Would running an external monitor with the 15" MacBook pro slow it down at all? Would it be able to handle a 4K external monitor? I worry about toting a computer around campus again that's my main system and running into issues like I'm having with my current computer, even though it's seen many miles.

 

I'll have to check it out, I was just hoping to avoid buying an outdated rMBP before school this fall. I've tried to keeping up with fall hardware rumors, but haven't seen much on this end. Any suggestions on what I should do?