1) Choose the colour that you like. Both materials should not be treated as either a chopping board or a place for hot pans. Both are more resistant than laminate but it is folly to belive that just because they are mineral materials, the shiny surface will not mark if mistreated.
2) Both carcases and floorboards will be strong enough (if the floorboards are sound)
3) �1550 does indeed sound cheap. First check out the quality of the actual pieces they will use. You will appreciate that granite is a natural material and the appearance of the patterning in the material varies. The best quality material gets sold to the highest bidder. However a very large cost element is the labour of getting the pieces exactly right. Conventionally this has been done in the past by sending a man out to make a template of the pieces required, then the pieces are prepared in the works, and the fitting team return to site. If one of those trips can be saved, saving at least 0.5 days in labour. If technology has enhanced so this specialist equipment all fits in a large van, it is going to help.
As always the best advice is to ask for previous customer references, and go and see the products in situ. There are huge margins in this line of business but large fixed costs to be covered. They must be very productive to be able to do it at such a price.