I'm a bit confused here, the MacBook Pro you have listed is an older model (2012). Is this correct? If so is the system new or being sold as used or refurbished. Depending on the seller you may void your warranty.
Please verify your system here so we can give you the best answer on what and how to upgrade your system.
A SSD may not be the best solution for you as much as I love them they do have there limitations.
Also, can you give us an idea on what you plan to do with your system, that would help.
I would go with a Seagate SSHD drive Vs a SSD unless you are just surfing the net. 512 GB SSD's tend to be on the small size for doing much. Remember you want to have about 1/4 ~ 1/3 of the drive unused, larger space for smaller drives.
As to RAM, if you can swing it I would go with a matched pair of 4 GB modules (8 GB total). That way you get interleaved memory which is faster.