Replacing wooden fascia board sections and gutters/downpipes

Ectophile

You can get plastic "wood" now that looks very much like real wood, and can be sawn or drilled like real wood.  It's usually a hard outer skin with a foamy interior.

If the gutters are old, they may be brittle.  When I had my fascias done, the firm that did them didn't even want to try reusing the old gutters.

Tucosalamanca

Materials are relatively cheap. For the cost involved, I would replace all fascia and guttering with uPVC, perhaps introduce some eaves trays as well?
If you can cut out rotten sections and replace to a reasonable standard, you're likely capable of replacing all of it.
It's not a difficult diy job, particularly on a low level extension. Youtube is your friend.

Albermarle

You can get plastic "wood" now that looks very much like real wood, and can be sawn or drilled like real wood.  It's usually a hard outer skin with a foamy interior.

It is basically the same product as the white uPVC boards used for facias, soffits etc. which also  has an outer skin and a foam interior and can be sawn and drilled, screwed into etc .
The 'wood look' is mainly just a cosmetic change. Sometimes they use woodflour mixed in with the uPVC, but not sure this would withstand the weather as well.
The reason that most UPVC products, including windows are white, is that the white pigment used is very UV resistant and using different colours, finishes etc either reduces the weatherability, or increases the price, depending on how it is achieved.

For the cost involved, I would replace all fascia and guttering with uPVC, 
It's not a difficult diy job, particularly on a low level extension

OP - The advantage of the uPVC for installing is that it is lighter than wood ( the interior is rigid foam) so easier to handle.