I'm afraid I'd have to ask you 100 questions before I could even attempt to help, Barque.
If I were actually there, I could have a look around and answer them easily.
We're often asked on here for rough guides, but it is really quite impossible. Try to find a local builder or two to take a look. It really is the only way.
I know it's difficult, but look for someone who will take a little time over it. Do not rely on someone giving you a figure "off the top of his head".
(I really hate that expression. I get it all the time at parties!)
Go on then............
Allow 15k for a (very) basic job.
25k for a full-spec good quality one.
It's a very easy job to do badly ;o)
.... and don't forget the 20% VAT