Immigration Situation at Narita

We are arriving at 7.30 am and would like to catch the 8.50 am Narita express to the City. Will it be too tight or should i book later tickets (say 9.15 or 9.45 am). I have read earlier posts which said it took 2-3 hours to clear immigration which is surprising.

You shouldn�ft book any ticket for NEX. Just buy it upon arrival.

I agree with LikeBike. NEX (Narita Express) departs from Narita Airport almost every 30 minutes, through morning to night, and you can choose any train regarding your arrival time. So there is no problem to buy a ticket upon arrival, and you don't have to make reservation in advance.
It'll be much better than wasting your reserved train ticket by any inevitable reason (e.g. delay of your flight, congestion at immigration gate).

Buy on arrival. You have the choice of four train services. The N'Ex is good for Tokyo or Shinjuku, but the Skyliner is better for Nippori or Ueno. Depends where you are heading (and that is why the local trains are useful as well).

As others have mentioned , you can just buy Narita Express upon arrival after you cleared customs.