An interior door frame is something that DIY'ers can usually handle. Do you intend to replace the door and frame? If so, and it's a standard size (usually 32-inch for bathrooms) you can buy a pre-hung door that fits right in, complete with frame. You'll likely have to do is shim the unit so that it's straight and square before nailing in place, and then pretty up the walls and floor if drywall, paint, etc. are damaged. Finally, paint or stain the new unit.