I would suggest you do all 4 lines at the same time. If one is rusted they probably all are.
You can buy pre-bent lines that should be easy to install. They ship rolled up in a coil, but it is easy to straighten them out and usually the bends are marked so you don't unbend something you need.
You will have to bleed the brakes no matter what, and Mr Blaine will yell at you if you don't change the brake fluid while you're at it.
There are lots of videos on Utube on how to bleed brakes, check a few out so you know what to look for before you start replacing lines.