It is always better if you have a professional do the job for you in case of any unevenness. They’ll make it fit properly.
Yes, contractors are very much capable of hanging doors. However, you want to make sure they are willing to hold liable for any damage first.
Hi, depends on how much time, money, and energy you want to spend…and how skilled you are with this type of stuff. If you need to replace the frame, than it’s not an easy thing to do, so I would recommend a professional. The best part about hiring a pro, is that its guaranteed and a lot of times you have a warranty (just don’t hire someone unlicensed)
If you do want to do it yourself to save money, and you’re capable of doing it, I suggest researching online or look up videos on youtube on how to hang doors.
If you have more questions i’m happy to help