I went through this a few months ago when my hard drive began acting up and I got a new Mac: The Mac is my sole work computer.
Lots of discussions on how to do it:
Mac 911: How to erase your Mac's hard drive the right way | Macworld
I ended up replacing the hard drive and the old unit works perfectly. I, uh, used the old HD for ballistics testing
Think about replacing the battery. Do a Time Machine b/u first. Replacing the HD was the first time I had opened a computer up and it was surprisingly easy. Folks here convinced me to give it a try and I am glad I did. The battery is also very accessible and is plug and go.
https://www.amazon.com/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Amacbook%20replacement%20battery
If I can do it...I now have two mirrored Macs, one that I travel with and one that remains at home.