I do tiling, painting, gardening,general building and handyman jobs.
Yes you can wallpaper over paint, just need to sand it before painting.
Some tradesman charging for call out, as they spent money for the travelling. some is not. they can add that later in the quote. it is up to you, who you would like to hire.
You can try to sand it down with 80 sanding paper.
The colour chosing usually is up to your own taste. but you can do them natural wood colour or same as island.
You will need a good builder, who knows carpentry. also yhe best to have electrician, for your own safety. but some builders can do this jobs, if they are not major.