Since the ceilings are 1.8m high this property is worth significantly less than those with average height ceilings. If this was a loft conversion it would have to have 2.2m height. It is very difficult to market (for sale) rooms like this because most people will stoop when walking in them. As a result some mortgage lenders will not class it as a habitable room and therefore not be prepared to lend on it (or will lend less because they think the habitable square footage is much less than the estate agents details)

So sound proofing is NOT your biggest issue. Ensure the price reflects this height disadvantage so 10-25% less than market average.