from what I can see, your HDD is over used, meaning you do not have much scratch disk space left.
your system disk is not suppose to store all the works and your files.

you need to start to work on your files and store in externally. then you existing system HDD will have sufficient space to work and will remain as fast as your first bought your mac.

as for external HDD, 3.5 inch or 2.5inch. it depends on what kind of solutions. 3.5 inch definitely need to power up.
as for 2.5 inch, some need to power up if you are using a bigger capacity HDD, however, home HDD enclosure doesnt need because the USB cable has 2 ports to connect to your mac to power up. it depends on architecture of the enclosure.

I have an enclosure for 2.5 inch, if you use the eSATA port, then you need to connect to external power.

if I am confusing you, then please give me a call or you are welcome to our office.