The only thing I would think that you'd need to mount an ssd is:
1. A place to mount it: Many cases include HDD racks that can accept both 3.5" and 2.5" HDD/SSD's, while some include a 2.5" to 3.5" adapter of sorts.
2. A Sata Data cable
3. A spare SATA power plug from your PSU
4. The SSD itself, screws are usually included in the SSD.
For budget conscious SSD, MX100 series are pretty good, though they may not perform as well as 840 EVO at 128 and 256GB capacities. SS SSD's usually perform better than the rest at lower capacities, at 512GB or 1TB, the differences are usually minimal.
I have both 840 EVO 256GB for the OS, and 512GB MX100 for games, both work as intended.
One thing that could alleviate your OS ssd usage is move your...