When i'm driving down the road i hear a hard knocking sound in the engine and the car runs rough and I have no clue whats wrong with it. What should i check to see what is damaged and what parts should i buy?

The engine started knocking and running very rough. Mechanic said we had to replace the engine. Also, the transmission seems to be slipping, probably have to replace that too. Had to park the car, can't fix it, can't drive it. Shoulda never bought it. Already replaced head gasket one month before engine went out.