Hello Noggin I have trained it from Agra to Varanasi about 4 years ago, I took the overnight train but cannot remember how much it costs.
Tickets are easy to book at the train station, but it can be a time consuming task. If the trains are full you need to use go to a local travel agent and book there they will source you a ticket but it will cost you more. I must admit if I had more funds when I travelled in India I would have just used local travel agents as it saves loads of time.
At the station you will be hassled by porters offering to take your luggage and then carry it on the train when it arrives. The one we used at Agra was short sighted and boarded us on the wrong carriage; I never used a porter again after that. The train will be at the station for about 20 minutes before it pulls off so no need to rush.
I agree with Judith use trip advisor to find your hotel
The travel agent in the Hotel will do all the donkeywork booking the trains if you do not want the hassle.
Has anyone else who stayed in Varanasi had power cuts? Someone told me it was the state cutting the power off, as the town does not pay all its taxes.
Hope you have a good time, Alan