Advance deposit for Mount Kenya trek?

I don't know your full itinerary, but I would not advice it. If they need the money to book the 'huts' - Old Moses and Shiptons and Top hut(?), - why not do-it-yourself by paying directly to Mountain Rock? The amount of deposit payable to them (for the remaining services) will then be lower, and you will be spreading the risk.

The reason I say that is because Kenya has squillions of 'one-man-and-his-dog' tour operators. Some of them are in it well over their head, but they keep struggling and trying to make it work. If something went wrong, there is a possibility that they may not have the resources to be able to put things right; and it is highly unlikely that you will get your money back.... and, take it from me, there is no point of dreaming about compensation.

I don't know anything about Sana Trekking, so I cannot comment on their reliability. But allow me to share my experience:

I contacted a company called Derviv Tours & Car Hire. I wanted to hire a car while I was on holiday in Kenya, and they came up on the net as one possibility. They seemed to be ever so helpful, and always kept in touch by email. To cut the long story short, when I got there, they did not even have the car that they had promised (and which I had paid for), but a different make/model. They were operating from a seedy little office in a dodgy building somewhere along Wabera Street. I did not want to waste any of my holiday time, so I agreed to take it. But the car was giving me problems (it was quite a junk under the hood). I asked them to replace it, but they did not even have another car! Of course I demanded my money back - but it was in vain...

Regards,

Mwenyeji.