In answer to your most important question, yes of course you are mad - basic qualification!
It looks very interesting, and it depends what you are doing with it.
I like pin-the-tail-on-the-donkey, so I will estimate 350k +/- 150k, on the basis of:
1000 £/sqm x 300-400sqm if you put 2 storeys in.
The area estimate being based on a 5m eaves height x 20m or so length to each side x 5m width = 200sqm minus an overlap at the corner.
Things that make a difference:
Is it listed?
How much are you doing yourself?
What are your skills?
2 year or 6-8 year "while living in it" project?
Specification, and if you control yourselves.
You need to be prepared for multiple surprises that will cost 10k+ each to sort out.
The "phase it" advice is very good. I might suggest design it so you can treat parts of it as separate to rent out while you are doing the other bits for income, or even 2 dwellings if you need. You could eg start with your kitchen and living in one part, then finish the bedroom wing in a phase 2 in 10 years time.
Find some good storage space, so you can buy "10% of new price bargains" several years in advance and store them somewhere well ventilated and dry.
I'm being a bit light-hearted above, but it looks like a really interesting project.
And consider buying a digger :-).
Edited by ferdinand, 12 January 2016 - 05:28 AM.