It's all about having the right information to make a good decision. As others have said get a good survey and if there are faults then price them in. Leasehold ground rents, heating, maintenance and repair costs can all add up too. If a flat is cold, noisy and expensive to run then it...
The age of the property and type are very important here. A purpose built flat will have be very different to a house converted into flats. Old properties, including flat conversions, with solid walls can be very cold and expensive to heat. Modern flats can be pokey with small windows but...