Travel insurance is one of the most important things you'll need overseas, particularly when trekking in Nepal. A policy that covers loss and theft, medical and hospitalization, evacuation from high altitude, and adventure activity coverage are among the basics you'll need. A wide variety of policies are available, so make sure to shop for the best possible options. You need to have a policy that's specific to the maximum height on your trek, bearing in mind that a number of companies place restrictions on that score — look closely at the fine print so that you're sure to get the right coverage, especially in the event of an emergency such as altitude sickness — helicopter evacuations are horrendously expensive.
Also note that most medical treatment and facilities in Nepal such as hospitals require payment upfront, so it’s wise to choose a policy that pays the bills directly rather than lumping the burden on you and leaving it till later for the insurer to reimburse you.