No, you don`t have to bleed the brakes if you change just the servo.
Undo the two nuts holding the cylinder to the servo and pull it clear of the studs, gently move the cylinder out of the way by allowing the pipes to bend, do not move it any further than you need to. Disconnect the vacuum hose.
From inside the car disconnect the actuating rod from the pedal, and undo the nuts holding the servo to the body.
You can then remove the servo (you may need to un-bolt the coil bracket and swing the coil and the bracket out of the way)
Before you fit the new servo check that the rod which pushes on the master cyl.
piston has the same protrusion as the old one.
I would get a servo from the breakers yard if I were you, new ones are very expensive and, contrary to your experience they don`t fail very often.
Colin.