frame rails

As Dave (fuzzface) stated, it depends on your skill level and availability of tools ( metal working tools, welder, etc....) Frame rail repair is very possible, it happens in restoration shops and by many hobby do-it-yourselfers all the time.  You do need some understanding of the construction of cars and how a repair vs. replacement of an entire panel effects the car.  The frame rail forward of the suspension is critical for sheetmetal alignment and part(s) attachement, plus some collision impact considerations.  The frame rail areas supporting the engine or suspension or center of the car is much more critical. The area of your frame rail rust is multiple layered. The outer rail & then in the bend area is a inner reinforcement. When you identify this, your repair plan needs to consider removing a larger section of the outer metal in order to repair the inner panel first & then repair the outer frame rail panel. You can accomplish this same thing with the floor pans & firewall removed. So as you can see, a thorough assessment of the rust damage is needed first. Remove some of the rusted metal to see the extent of damages. Create your repair plan. Decide if you have the tools & equipment to complete the repair yourself or if you will send the car out to have a pro complete the metal work. Consider the cost(s) of each. 

Another thing to consider is the level of knowledge you need for repairs in this area of the car. A cardinal error is not setting the car up to be level, make measurements for all sheetmetal alignment ( door to fender, door to quarter, door to roof gaps (measurements) and then compensating for sag in the car body prior to removing and welding in replacement panels or patches to the torque box area of the frame rails. In other words when you start removing metal from the frame rails or the car has rust, the car actually sags ( not visable to the naked eye. sometimes it shows up as door alignment issues). Before you cut out a frame rail & weld in another replacement panel, a understanding of how to set the car up & measure for sag is needed.