I get around 35-38mpg if i drive my 16valver steady, but if i have my lead shoes on that drops to 30ish, if not less, so it depends on how you drive it.
I dont think the VR6 will cost you that much more to insure.
And yes it is Faster than a 16valver! in both acceleration and top end speeds.
I hve no idea on servicing costs, but would personally stay away from stealership servicing costs, and find a good competent local garage.
Hope this helps, and good luck searching for for mk3