I bought a refurb mid-2012 13", and ordered RAM from Amazon and OWC Mercury Extreme Pro 6G 480GB SSD from macsales.com. Youtube videos for the hard drive and RAM videos helped, but I was comfortable opening up the case (OWC furnished tools for the job with the SSD purchase). Simple task for both the RAM and drive swap. I opted for external enclosure, but thinking of buying the adapter to pull out the optical drive to have two internal SSD drives. It turned this 5 year old tank of a machine into a performing workhorse.
For the cost, you can't really get a better MacBook Pro to upgrade, and with integrated graphics, you won't be at risk of the failure that sometimes happens with discrete graphics.
The only issue you might encounter is, if it still has the stock 500GB hard drive, is an issue with the ribbon cable. It was not an issue for me (I bought a refurbished unit), but if you run into the problem of the SSD being slow or not recognized at all, it could be the ribbon cable, which can be bought for $11-$15, probably.
I'm running Sierra on mine, and it does what I need it to do (music production) and does it pretty well. I am a happy camper.