TBH with 6GB RAM that should be plenty for games etc , I would go for CPU first or SSD second or both if you feel rich
Plus when changing CPU it will allow you to clean out dust and debris from heat exchanger ( radiator ) that you normally can't get to , remember the hotter Laptop runs the more the CPU will throttle its power to keep itself safe , so a hot running lappy will naturally run slower and slower.
CPU s/h on eBay probably £50 , SSD 500GB to 1000GB ( 1TB ) new about £100/150 , s/h SSD's are fine if you know it hasn't been used hard for writes ( like from a data centre or server farm ) most sellers of s/h SSD's will list info on the health of drive which will tell you how much use its had , s/h CPU's are fine ( these can only be damaged if majorly over clocked but the sort of CPU you are looking at I very much doubt anyone would take the trouble to squeeze that little bit of power gain that can be extracted from that model of CPU by over clocking )
Changing CPU , 1/2 hr work to physically change , windows should simply load any new drivers on boot .
SSD you would need to image the original drive onto SSD or load windows fresh , both take a couple of hours to do.