The only reason you would need insualtion in the floor in a room in the attic is to make it less noisy when walking up there, carpet and padding will do the same thing. If there is no insulation then the heat from the rooms below will rise and heat the room above.
As far as using foam instead of fiberglass batts, it would cost at least 10 times as much and would have to be done by a pro. Not a DIY job at all.
By using a Pro if they know what there doing may suggest removing all the old plaster up stairs so 2 X 2's can be added to the rafters so that foam baffles can be added up againt the roof sheathing to allow air flow then add R-13 or 19 depending on how much room there is.