Mongolia has become a good place to both convert your excess Chinese money and get hold of some rubles for Russia. Otherwise the borders have banks which will exchange your dollars at a reasonable rate. In Short - no. Variety and quality have increased in the last few years - some people even know what a vegetarian is these days. On the trains plenty of food is available on the platforms and, apart from the train between Ulaanbaatar and Irkutsk, the dining cars are usually well stocked and don't run out of food contrary to popular belief.
Fruit is available most of the year and bottled water is as common as vodka.
We'll be happy to answer any additional questions on your mind, though we are sure that most answers can be found while browsing through our Monkey Shrine.