Ebay (UK at least, I haven't looked at US) is full of used Webasto Airtops, and in fact the Webasto website specifically offers the Air Top for passenger bus heating (see link below). I've been tempted by the idea of getting an Air Top a number of times as they are much cheaper and simpler to install than a hydronic system - but of course while they will warm the bus they lack the ability to provide hot water.
Whether a new Russian heater is a better buy than a used Webasto for similar (or less) money I couldn't say. I'd want to research the Russian unit to make sure it met the relevant standards, but I probably wouldn't be worried about it too much beyond that. My preconceptions say that the Ruskies probably know how to make effective and reliable heaters, but it might well be quite crudely put together.
Webasto Air Top:
Air Tops for buses:http://www.webasto.co.uk/general/en/html/8348.html
PS - As it happens I bought a new Webasto Thermo 90 last week - anyone want the unused Webasto Top 'C' that I bought previously but decided probably wasn't powerful enough for the bus?