the old ps3s dont overheat quickly, just change the thermal compund, the fan and the PSU and you will be fine.
Im going to elaborate. The older ps3 have a design flaw of which sony kept secret. They intentionally used a thermal paste which sits on top of the CPU and GPU. The reason I say its a defect is because originally the ps3 cost 599 dollars and the thermal paste we aretalking about last around 2 years before drying out. Yep not a long shelf life for a console huh????
What happens then is something wonderful. The ps3 gets so hot that the CPu and Gpu overheat and pop off of the motherboard. Once this happens you get the red light of death. The console is now broke. OPtions are to get it re-flowed or send back to sony is waranteed.
If I was you wait for the ps4 and pray its better....Another reason I say this is because you can't play ps3 games on the ps4 for its not backwards compatible so if your wanting a new console don't buy an old one.