Train Travel

If you can commit to a specific train and date, it is always worth booking tickets in advance as long as you know that they are unchangeable and unrefundable, unless there is a strike or services are suspended under a weather warning.
I would also say to choose a seat on the right hand side of the train, for the coastal scenery (although that doesn't come until well north of Newcastle), of Durham Cathedral, and of the Tyne as you approach Newcastle.
Whether first class is worth it is a subjective opinion. To me the answer is no.
What you get is use of the 1st class lounge at KX, a wider seat, cars which should not be subject to overcrowding, a two course light meal, tea/coffee/soft drinks throughout and alcohol with your meal (only). Is that worth typically £60 to £70? Your decision, as your valuation may be different to mine.
The other option is to buy a standard class ticket then bid for an upgrade on seatfrog on the day.

Buy as early in advance as you can using an Advance ticket. First class is up to you but it's generally a waste of money in the UK