You will have many chances to have medical problems whilst in Nepal, prevention is better than cure so make sure that you are immunised against the common diseases and take a medical kit with you. If you go on an organised trek there may be a communal medical kit so check beforehand. Also make sure that your travel insurance includes being helicoptered back to Kathmandu which can be very costly.
I made sure that I had been immunised against Hepatitis, Polio, Tetanus and Typhoid and took the following items in my medical kit whilst on trek :-
As far as water is concerned you may be able to stick to bottled water if you stick to the popular routes. This is expensive and the plastic bottles are an environmental problem. I only used iodine tablets in Kathmandu and drank boiled water during the day supplemented with cups of mainly lemon tea from the "restaurants" along the trail. I used the bottled water as an emergency supply.