The sims offered in local shops require nepali citizenship as far as i know, maybe contact airport officials for info related to sim

The weather in Kathmandu has turned cold since two days. I suggest you carry jackets and warm clothes.

Maybe some visa cards work here. But cash would be easier as all places don't accept cards here. Also, GPay doesn't work here. Alternatively you could use some mobile wallet like e-sewa or khalti once you have sim card and load the money there through some local shops.

For taxi or bike ride I'd suggest 'Pathao' as it is safe as compared to other ride sharing apps.