Your first fitting should not be until 6" above the floor level rim of the highest fixture served. This would be your lavatory. A no-hub clamp connection would not be a week point. You can install bends in your vent but the piping should grade downward. Reconnection to the cast iron at the roof is okay. You may be able to reduce the size of your vent but it depends on the total vents serving the house. You have to have the same vent area as the house drain which is four inch. If you have two other two inch vents you may be able to reduce. Schedule 40 pipe is the same size outside diameter. You cannot use thinner wall pipe. PVC (white) and ABS (black) are the same size.