I agree with johndoh.
To basic build out the ground single storey, walled, roofed, leccy, plumbing and pastering I’d say the £1k to £1.5k is about right. That wont include the painting, carpets and furnishing which will all add on more … but personally, I consider that a secondary worry. I’d rather spend my money to be sure I had a solid room before I worried about buying a carpet to put in it. The room will last longer than the carpet / decor.
As for the refurbishment, again, wholly agree with johndoh. If you buy a bathroom suite for £500, tiles for £500 etc and think ‘thats the bathroom cost covered’ then you’ll be surprised. All the add-ons add up. The adhesive for tiles, the plumbing connections, the sealants, the towel rail, the tile cutter, the daft little tile spacers, the lights. You regularly find yourself popping out to the local hardware store and coming back £30 to £50 lighter in pocket. Those trips quickly add up to hun’dreads’ of pounds.
I’m a believer that if you go into a house and get a good ‘feel’ for it and you have a gut feeling that ‘I want to be here’ then go for it.
Decor, crap wall paper, avacado toilet pans can all be changed. Look past them and see what i can be. Good luck.