You may need to remove the bottom course of tile/slate first(if slate best to remove two course).If its tiles and your lucky you MAY just be able to push the bottom course up under the second course.Alot easier with two men and yes you need scaffold for such a job.PVC fascia you want the 18mm stuff,not the thin cover board.Make sure your spar feet are solid as you need a good fixing to ensure it stays put.More often that not you will need to replace a strip of sarkin felt,to be fitted under existing.You can now get plastic eave strips for this if you prefer,if using this put silicone on the laps.Also get the nails with large heads for your fascia and small heads for your soffit(thats what i prefer anyway) .Fit your corners and jointers with nails(small headed) not silicone or glue
good luck.ps,any other colour than white is very expensive for fascia/soffits but not so for gutters/downpipes they are priced equally.Also buy your pvc products form a pvc supplier,if you go to a builders merchants you will pay well over the odss. 
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