Rotors don’t rust. Surface rust yes. get a new mechanic.
One of these sentences is not like the other.
Of course rotors rust. If they are vented they rust between the two discs (in the air gaps). Good rotors are dipped in paint by the manufacturer before the pad surfaces are machined to prevent this. But many cheap rotors are not and if driven on salted roads they can develop severe rust issues.
After paying big bucks in my younger days for pads and rotors, my brother in law showed me how to change them.
Damn, did it piss me off. If you have minor mechanical ability, it is not a hard job at all. You know that drunk neighbor that is always working on his truck? (sure you do, we all have one :)). Buy him a 12 pack and have him show you how it’s done.
That varies with the design. While many are fairly straightforward and relatively easy, not all are. Some older Honda models require removing the wheel hub from the steering knuckle, which is a fairly major undertaking.
Just don’t let him do the actual work on your car. 
Or some Audis, or rear brakes on a Jag.
Hey, on many smaller FORD models, yo don’t have to remove anything to change the brake pads-they slide up through the calipers.
That’s the kind of design I like!