Go solid state. I recently switched the 7200 RPM in my Mac Mini to a solid state drive and the difference in speed is phenomenal! It boots up in a matter of seconds now and file access speed is much improved.
Also, there are many external storage solutions that can hold the projects you are not immediately working on, so you can free up space on your internal drive.
Yes. Or choose something like the Jottacloud backup plan - they also have an included online archive.
I don't trust anything but WD. I would never touch a Seagate drive- super high failure rates...all risk