While you may be able to use ATM and credit cards as far as Syabru Bensi, we encourage you to carry cash with you as with increasing altitude, you will not find such services. Keep in mind that the expenses go on increasing the higher you get. This is due to topographical challenges in importing goods at such altitude.
You will need money for drinking water, charging your devices, internet use and phone calls and your personal expenses. For these, around USD 300 should be enough.
Note: Don’t forget to change your dollars into Nepali currency while you are in Kathmandu. You will find plenty of money exchange services in the streets of Thamel.