Getting from the plane to the train

Oh...also, would it be reasonable for there to be remaining seats without booking.

Hi Nat,

I don't see a problem with traveling to Sendai if your plan lands at 18:45 at Sendai Airport.

It is however not possible to buy train tickets outside of Japan. However there's no worry about seats selling out. This rarely happens and only during national travel periods.

Hope this helps,
Daniel

Case 1. when you are eligible for JR East Pass, the best train you should take is JR Narita Express 50 departing from JR Narita Airport station at 1946.
Case 2. When you are not eligible for JR East Pass, it is cheaper and faster to take Keisei Skyliner. The last train departs at 2043 from Keisei Narita Airport station.
Case 3. when you are eligible for JR East Pass but your flight is delayed, you might have to pay additionally to take Keisei Skyliner, and then take JR Shinkansen with JR East Pass.

In short, whether you can use JR East Pass(you must have a temporary visitor visa), and whether your flight arrives on time matters.

http://www.hyperdia.com/en/cgi/en/search.html?dep_node=NARITA%20AIRPORT%20TERMINAL%201&arv_node=SENDAI(MIYAGI%2FJR)&via_node01=&via_node02=&via_node03=&year=2015&month=02&day=27&hour=01&minute=32&search_type=1&search_way=&transtime=undefined&sort=0&max_route=3&faretype=0&ship=off&lmlimit=null&search_target=route&facility=reserved&sum_target=7

http://www.jreast.co.jp/e/eastpass/