Before you start pouring money into a system worth less than $50, carefully evaluate the cost of the parts against the cost of a newer, complete system.  Any 2.5" SATA SSD will work.   With the battery, you're going to be limited to the aftermarket, as any OEM batteries for a system of this age will be many years old before you even start using them.   The reserve battery is a CR-2032 coin cell with a pair of wires and connector attached. [URL]  
Hello, I just received a Dell Latitude E6520 without battery and hard disk drive which also needs replacing the bios battery. As I'm not into computers at all I would like to have your confirmation for the following so I won't buy something that's not suitable for it. Been told by the donor that it would work well with a Samsung 500GB SSD, a new battery obviously and the cell for bios. What type of battery cell is needed for the bios please? Any specs I should know for the battery? Like number of cells and amps? Can I also do any upgrades to make it run slightly faster?   Thank you in advance!
Dell Latitude E6520 upgrades