You are probably back already, but just in case somebody arrives here, I would like to suggest a local company I always use: nuracar (dot) es
They have always been really helpful. I do usually visit Menorca twice per year and they have been my first pick the last times, and will be from now on as well.
The cars are pretty new and in perfect conditions, the staff is really nice and helpful and prices are as announced, no extra charges, etc. They do even offer you the possibility to pick the car at the airport and return it there without extra charge, although I would say they don't have an office there.
So if you are travelling to Menorca, that's my suggestion!