Hi there,
Yes SSD and RAM are the way to go for laptop upgrades.
I think a CPU upgrade is possible (depending what you currently have) as the laptop supports mobile Athlon II X2, Phenom II X2 and X4 CPUs. The top tier CPU is the AMD Phenom™ II quad-core mobile processor N950 which can be bought for ~£70. Though please bear in mind that CPU upgrades are generally much trickier than upgrading the RAM/SSD on the laptop or replacing a desktop CPU.
Here is the service manual which shows you how to disassemble the laptop, page 116 shows the CPU replacement.
For the graphics, the standard 5552 only has onboard HD 4250 graphics, so no GPU upgrades are available for this one.
As for the SSD, the laptop uses SATA 2, so the super quick SATA3 drives wouldn't be fully utilised. But to be honest, for most operations a SATA2 connection won't limit the performance of a quick SATA3 drive, so I would just be looking for a good value, reliable SSD and don't worry about whether it is SATA2 or SATA3. Personally, I would go with
this one. Also, bear in mind that this laptop uses standard height 9.5mm hard drives, so you don't need to go for a thin 7mm model.
As for the RAM you will want a 8GB (2x4GB) DDR3 1333MHz kit, something like
this.
Finally, the bay that converts the optical drive to a 2.5in HDD/SSD mount is one like
this. The laptop uses a SATA connection for the optical drive, so it should work great.