Mid 2012 MBP Hardware Upgrades

To start with I would bump the RAM to 16GB which is the systems limit Замена оперативной памяти в MacBook Pro 15" Unibody (середина 2012 г.)

As for as drives I would go with a 1TB so you have plenty of space. I would go with a SSD Vs HD as its just more rugged and uses less power Замена жесткого диска в MacBook Pro 15" Unibody (середина 2012 г.).

The one issue you do face is the HD SATA cable will likely need to be replaced if you still have the original HDD in the system as the original cable was not able to run at the higher SATA III (6.0 Gb/s) data rates the SSD will push without encountering issues MacBook Pro 15" Unibody (Mid 2012) Hard Drive Cable I would also apply a strip of electricians tape on the upper case where the cable crosses over and explained here Your Hard Drive Cable Is A Ticking Time Bomb

The last issue is making sure you don’t damage the cable installing it. A common failure I see is people crease the cable which damages the thin foil wires with in the cable. I use an old BIC pen ink straw as a bending brake to form the bends so I don’t exceed the radius of the straw.

I would leave the optical drive alone as it needed for structure and with one large drive you should not need a second.