1) I did this unsuccessfully at first but while going through the process managed to learn how to properly wire things.
2) Depends on the area you live in and the type of house you have. Single family homes are usually more flexible in permits than condos or townhomes with shared walls. I did not pull a permit for my bathroom fan but likely was supposed to based on my city's code website.
3) Depends on the "pain in the ass" factor of getting into your attic to run the wires and exhaust duct. I found it fun to learn more about all the quirks of my old house. I would happily install another bathroom fan but would consider paying someone if the bathroom was close to the edges of the attic. I don't enjoy laying on rafters when there is very little working room.
Some thoughts.
1) Please please please duct your moist bathroom air out of your attic. You don't want to just move your moisture problem to a part of your house you rarely look at. I cut a hole into my roof and ducted the air outside of the house. Works great 2 years later!
2) Get the power for the new fan from the nearest "home run" that should power part or all of your bathroom. The location of this depends on when and where your house was built. You can add a junction box in the attic if the old junction is buried in a wall somewhere. Part of the reason you'll need to find the power away from the switch is most switches are just a break in the "hot" power cable. To wire the fan you'll need hot, common and ground. Sometimes all of those will be in switch box in the wall but that's not a certainty in older houses. Home runs are usually a romex cable or conduit line that is headed in the direction of your breaker box. You can run power from the junction box, to the new switch for the fan, back to the fan. This would let you either add a separate switch for the fan or one of those fancy timer switches.
Also always turn off your breaker AND use a non contact voltage tester before you touch / cut any wire. 120V 15amp likely won't kill you but it hurts more than you'll like.
As for difficulty I would say this is a moderate task. Not as easy as replacing a switch but not as hard as trying to add a window. I would give yourself several evenings to educate yourself and 1 full weekend day to complete the install.
