New door frame and door required do I need a builder?

We have an old cottage with a thin front door, rim lock and ancient massive iron key. The door has gaps all around it so it's noisy, drafty, and water comes in when it rains heavily and cars go past. We thought about weather proofing and the carpenter said it was possible, but we would still have a thin front door and the weather proofing would probably not fill the huge gap at the top without looking silly. We want a new thick door with a proper lock. How do I advertise on here, there doesn't seem to be an option for a builder to deal with door frames?!