A safe tech would be where I would start on the lock. If you feel like you can take the lock out and drop it on the shop counter might save you a bit of money. At least it does here in the states. You can also might save if you mention that you are in no hurry.
As for the restore part. I would look at it this way. If you find that this is an important safe to you then it would be somthing to restore. If you are only looking at it for the value then you need to find out if this safe was made in few are great numbers. If it is that this safe is rare then it is somthing that should be restored. To me the true way to do it is find the paint that was used then and get that. Sand blast it down to bare metal and prime and paint. Get the lock serviced so that is torn down and cleaned and lubed with the new key. To me it comes down to this: I find a lock or somthing I find intresting and needs my help to come back to where it was made to look. This takes time. Some will not buy a lock/safe that has been “restored”, but in cases like this it is the only way to stop the safe from being a pile of rust.
It might help to get it inside.
