The current rolling stock on the line is ex British Rail and is nearly 50 years old.
Try halfing that. Im not sure if the line has Pacers or Sprinters. Pacers are about 25 years old and Sprinters 20 or so years old. They are based on older technology though (Pacers especially) but nothing like 50 years old
It is owned by a private company .I believe it is Northern Rail at the moment and is passed on whenever the franchise changes hands.
The trains are owned by a leasing company, for Northern Rail's diesel trains it will be either Angel Trains or Porterbrook
The rolling stock ie the Metro trains would have to be bought by whoever owns the franchise at the moment and until the trains finally die there is no chance of that.
The stock would most probably be bought by a leasing company, and if there was a guarentee the stock would be needed long term Im sure they would be interested in purchasing new stock