Hi,

yes you can buy UPVC capping fascias and soffit boards, however the original timber would have to be in a good condition with no signs of rot, if it is rotted then the timber should be cut out and replaced.
Once the timber is covered with UPVC this will add a degree of protection to the original fascias. Capping over is a cheaper option than full replacement but from down below you cant tell the difference so it still looks nice.

I personally wouldn’t as if the wood is rotting or anything will affect this in time so I like to do this properly and remove all the wood and replace with upvc

Hi as the other guy answered
yes
you can but may be better buying new guttering if the original guttering is old and brittle?? and if you have rot you may as well remove the wood instead of repairing as time and cost will add up and just replace with plastic fascia and soffit wil have a longer life too

No , You should never clad over ( put plastic fascias over existing timbers). The timber will rot behind. Replaced lots of jobs and the timbers behind can be in a very bad way.
Do it once and do it right.
You will need some kind of vent as it needs to breath. Perhaps look on the Internet and see examples. Hope this helps