You may find it cheaper to book to White Hart Lane, this will include travel between King's Cross & Seven Sisters on the Victoria Line. Northumberland Park can be useful as hardly any of the crowd go there compared to WHL.
You will be lucky to be on the 18:00 from King's Cross. Do you know how bad the queues can be at White Hart Lane station? It's a long walk (30 mins but allow more, bearing in mind crowds) to Seven Sisters or Tottenham Hale but probably worth it as there will be less queuing. I would not buy Advance tickets for the 18:00 though unless you want to leave the match early! If you miss it your tickets will be worthless and it would cost an additional £40ish to come back on the 19:18 or £90ish to come back on the 19:00.