Thank you all for the useful and encouraging messages. We've never undertaken such work before and we are a bit concerned that it will be too much for us. We already had a builder to come and see it and he gave us a quote for the kitchen extension and the staircase, around 20k for the kitchen (top range materials and finish but not the kitchen units themselves) and around 7k for moving the staircase (with which I was positively surprised, as I was expecting it to be much more). But, of course, it needs all this other work that you mention, e.g. doors, moldings etc.
Caroldecker, I looked at them again and they are exactly the same in terms of size across the three floors (4 including the cellars). The second does not have a 3-storey extension but it seems to have a bigger garden, which of course is important.
Inkblinkandmustard, I did not know that the house had been on the market for long. We only started looking at Didsbury a couple of months ago and the first time we noticed it went up on the market was last January but at a higher price (375k). I really hope this has to do with the fact that it was tenanted and that the layout is strange (although hopefully this can change). Do you remember, by any chance, the price at which it was on for when you looked at it?