1) What kind of case do you have?
a normal one, nothing fancy ,you know it holds everthing that i put in . here see for youself
Wow! Look at the dust around the CPU heatsink in that first picture
Get that thing cleaned out big time mate and reapply thermal paste to your CPU also. Then check your temps and report back
And yes, get 2x 120mm fans (One for thr front and one for the back) and make sure they are positioned correctly to create the airflow. ie, Front inwards/back outwards.
Just to add i clean my PC out at least once a month. I use a soft clean (new) paintbrush along with my hoover. Some people don't recommend using an hoover as if you're not careful you could easily damage one of the components (ie. pots). But i have done it this way for years now and never had any issue's. At least this way i am sure i have done a good job. I even go so far as to pull my PSU out and open it up (Every couple of months) and give that the same vigorous treatment. Oh! And i allways wear an anti-static strap
Good luck and i hope you get it sorted