Should an old window be repaired or replaced?

Start with the fault. Hardware problems — handles, hinges, rollers, seals — are frequently repairable if parts are available. Frame problems such as corrosion, distortion or rot point towards replacement. The condition of the frame usually decides it.

Sometimes. Some hardware is generic or has a compatible modern equivalent, and some older systems have no parts left at all. Identifying the system determines whether a repair is realistic, which is why the existing hardware is inspected first.

Yes, where the frame is sound and only the sealed unit has failed. This is common with misted double glazing. It is not worth doing if the frame is corroded, distorted or no longer sealing, because the same problem returns.

Not necessarily. Heavy operation is often worn rollers, a damaged track, or debris and salt in the running channel. Rollers and tracks can frequently be replaced. Replacement becomes the better option when the frame itself has dropped or distorted.

Yes, and staging the work over time is common. Plan the colour, frame material, opening style and glass specification at the outset so that later phases match. Deciding room by room tends to produce a house that does not match.