On which basis should I decide for a travel insurance for Nepal?

In the end, I took Ergo Direkt (going by averells answer). And I needed a helicopter rescue, although having had several acclimatization days and a guide. Insurance-wise, everything worked fine.

How it worked

1. I signed the contract (online, no real "signature")
2. They send me lots of paperwork, including my insurance number.
3. I wrote down the insurance number and the name of the insurance in my "travel book". I left everything at home, except for this number. But this I put in a spot where I could find it easily.
4. I was struggling the day after Gorak Shep. I was not feeling good the days before either; the cold nights, a sore throat and stomach problems made me feel really shitty. My hiking partners told me I was stumbling all the time. I had a REALLY hard headache. At some point my guide (Ujjwal Rai, nice and experienced guy; speaks English fluently and learns Spanish) told me that I need to go back to Kathmandu. He called somebody (the helicopter company?). They wanted to know my insurance number. We finished the day trip and went to the helicopter landing site.
5. The helicopter took me to Lukla and then another one to Kathmandu. My guide arranged it that I went to a central hospital (ERA Hospital - they took great care of me and the medical staff spoke English fluently).
6. Within the hospital, after a first initial screening, they wanted to see my insurance card. Which I didn't have. After saying the name of the insurance and showing them my note with the number, they called a more senior doctor. He heard the name and said something like "that works". I felt super insecure at that moment. I wished I had taken the page with the insurance number with me.
7. The next day, the insurance called me. They asked if my room / the treatment was good.

What to learn from this:

1. Ergo Direkt is good; stick with an insurance that might be better known.
2. Take the paper with your insurance number with you!
3. Maybe write down the number of a helicopter rescue company / the number of the ERA hospital. Or make sure your guide really knows what to do in case of Emergency.

What I got

1. Helicopter rescue: This was likely the most expensive part
2. 3 days at hospital: A single room and 3 meals a day. The room was nicer than my first 40 USD/night hotel room.
3. Medicine + medical consultation: On my 3rd day (in Namche Bazaar) I had severe breathing problems. There is a health post. I paid in cash (USD, NPR and VISA was accepted, not master card!). Turned out the breathing problems were related to my stomach. Who would have thought. I later send the documents / the receipt to my insurance and got the money without and problem back.

While I was waiting in Lukla for the second helicopter flight to the hospital in Kathmandu, I've heard a pilot answer a request for a luxury flight. He wanted 1500 USD for a flight from Lukla to Gorak Shep and back. Just that you have a ballpark estimate how expensive that rescue flight might have been. I'm really happy I was insured. And really happy that Ergo Direkt turned out to work that well.