Hi,
I used to have a Focus 1.6 auto like yours and sold it with 40,000 miles on it, didn't have any problems with it at all.

According to my magic little book (Haynes), it could be;
-low on transmission fluid,
-a fault with the transmission sensor or wiring
- or a fault with the engine management system.

I would not trust that auto specialist, so many garages will con you out of paying large amounts of cash unnecessarily. It would be better to find someone on trustatrader.com.