You are right you can buy a mobile SIM card upon arrival at the airport terminal also. But there will be many other peoples who are buying SIMs. So, it takes some time to buy SIM due to the crowd. I would like to advise you to buy a SIM card at Thamel/outside of the airport. When you give a copy of the Passport and PP size picture with a page of form fill up, you can easily obtain SIM Card at Thamel even grocery shop nearby your hotel, the cost will NPR 100 or more depending on how much data you would like to buy. There are two different Cell Phone company NTC and NCELL, for the Everest Trek NCELL is better due to good coverage. Now a day, there is cell phone reception entire trek even Mt. Everest Base Camp (5,360m) and Kalapathar (5,555m).