Steel is really expensive, kind of ugly if you get that side of the fence and still needs concrete to set properly depending on your soil type. You can get a good pressure treated 4x4x8' post, two 60# bags of concrete and 25# of gravel for less than just the steel post. Start with a 3' deep hole 12" dimeter for the line poles. fill the bottom of the hole with 6" of crushed rock. Go 6" deeper and 12" of gravel if you have clay soil that does not drain well. Then fill the hole with the post that has been tared up to the top of the concrete line centered and resting on the gravel. Pour the concrete 4" above ground level and dome the cement so rain water runs away from the post. Water is your enemy. Save the pickets from the old fence to make forms for a concrete curbing to run the entire length of the fence so the bottom of the pickets are never in standing water and you do not need to weed whack around it. Use 6x6x10'corner posts sunk another foot deeper. This framework will last 30+ years.
If you need to cobble shit together to hold up what you have, driven metal posts work well for that
Good luck

