I would definitely recommend getting all the wallpaper stripped first, secondly i would recommend get the full room plastered get all the really messy stuff out the way first as chances are there will be bits off plaster to sand off the skirting boards ect.. so after the plastering is completed i would them recommend getting your new door fitted and whatever else you need fitting with the door, as then at that point you are just left with the decorating side off the job which would mean your decorator can come in and do all the sanded and prep work that's needed on the woodwork and walls without worrying that another trade is going to come in and potentially get mess on the work he's already sanded down and prepped ready for paint then finally the new carpet can go down knowing that's the last thing to do and there's nothing left to be done on the job that can potentially stain/ruin your new carpet.