It depends what's missing exactly...
I had replaced my sunroof with a good rustish free sunroof: cost £195 plus another v£300 to get it bare metalled and painted professionally.
Are the runner still in the bus? long aluminium channels about 2.5 metres long?
Cables - are they missing? Your looking at another £250 plus for these and minor plastic crank, another £40
Sunroof felt - £60 from the States (over here its £100).
Sunroof cover (clips on internally (£80) second hand.
Then there are lots of other trim parts that need to be there, but I think these are not vital. Throw some pictures up if possible so we can se what's what.
Graham on here has done a few sunroofs, he's the expert.
It is an expensive business, it burned a hole in my bus budget!
LV