Depending what model. However, in my car I chased a leak from the evaporator both ways. The "sniffer" did not catch it. The dye, did at the bottom of the car, in the transmission tunnel.
As far as the hose, it is true. Most places where they make high pressure, hydraulic or A/C hoses can make/repair your old one for a fraction of the cost of a new one.
Make sure this people are familiar with Mercedes Benz.