In summary:
We are selling up and downsizing to a nicer town for financial reasons.
We are interested in two flats. I won't do links on here so I'll try to 'write' a picture 
Flat A: £175k, complete doer upper, we would would buy and refurb with proceeds from our house sale but have to live in it (with kids one of who is disabled) while the work is done. Lovely family area, lots of social housing, next to a nature reserve. We would still have around 20k in unsecured debt which we would have to pay off. Very low leasehold fee.
Flat B: £250k, could move straight in, smaller than Flat A, in the very centre of a lovely market town, near lots of eye-wateringly expensive housing so flats are all privately owned and filled with professionals with kids. We would port over our current mortgage, pay off the debt which is sinking us and have savings to meet unexpected bills. Leasehold fee £350 a quarter which we could afford.
Just to be clear, yes, we are sure about moving into a flat and no we can't stay in our current house.
Which would you choose and why?