As someone with little knowledge of cars, my thoughts would be that you seem to have more than one issue (excluding the brakes which I didn't read). If you have changed batteries and the battery still going flat then, to me, that indicates an issue with the charging part of the car. I'd have thought a decent mechanic would find why it wasn't charging when the car is running. But a non-charging battery ought not stop the car since once running it isn't needed. There must be a further issue for the car to suddenly stop. Possibly again electrically related.