as long as your new windows and door are not bigger than the openings you have now it should not be to much of a hassle. if you have to extend the openings to accommodate the new windows and door then that is much harder. it is always easier to fill in to make it fit than tear out to make it fit.
as for the floor, you could lay 2x4 or 1x4 material down flat over existing floor and then refloor on top of that. depending on which you choose it might bring the house floor level to the same height as the porch. If you seal in the porch good enough you could even change the front door over to an interior door and get rid of the huge front door and threshold, making it look more like interior space. or, when changing the floor just rework the front door threshold and leave the door.