Well you just cant buy a new drive and put it in there, There are different types and the Motherboard of the drive has to stay with the console as you will find out from previous post on here.
The blu-ray drive is married to the Motherboard, (keyed with the Motherboard) needs to be matched just as a Xbox 360 dvd drive is keyed specifically to each console. there for you have to do some research on the motherboards and type of blu-ray drive you have in the console. if someone changed it that could be a major problem already. "You did say though it Played games when you first got it" so there for i think the laser is just going bad and if your going to replace it you might as well replace the whole dock it sits in due to how dirty you said it was. being that if the laser is bad i am sure all the components that work the laser are pretty worn also. I have replaced a few myself , actually takes longer to take the console apart and the dvd drive apart then it does to replace what is wrong with it,