Any item left outside for long periods of time will suffer wear and tear. A well made item may have occasional problems that can be easily fixed.
Spotting the Common Problems
You should go around the railing in the order you found the problems and list them. Typically surface rust is the first sign of metal parts beginning to corrode, but that can develop into pitting, in which holes form in the surface of the rail, or looseness of the joints between sections, or even cracking of the welds holding the sections together. Of course, there is also the possibility that one or more of the spindles may have been torn from the base of the post.
Removing Rust Properly
A wire brush alone can only remove light rust; a disc or flap wheel on an angle grinder is a lot faster when dealing with large amounts of corrosion. Once all the rust has been removed from the surface of the metal, apply a rust-converting chemical treatment to the bare metal. This converts any remaining rust into a stable surface which can be primed.
Filling and Smoothing Pitted Metal
If there are any pitted areas, these need to be filled before painting. A two-part metal filler will do a good job of filling deep pitting, and needs to be fully hardened before being sanded down to a smooth surface.
For more on this, take a look at https://donkeywellforge.co.uk/what-we-make/wrought-iron-railings-bespoke-metal-gates.
Repainting for Lasting Protection
When painting wrought iron railings found in an outdoor environment, we would recommend starting with a red oxide primer on the bare or treated metal surfaces followed by two coats of an oil-based gloss paint. This combination is far better at preventing moisture ingress than water-based alternatives.
Welding and Replacing Broken Sections
Cracked welds and missing spindles are generally not something you wish to attempt as a DIY job. They are best left to a blacksmith who can re-weld the joints as required and even make new sections to match the original profiles. .
If you tackle the repairs a bit at a time your wrought iron railing could last for many more years than if you replaced it in its entirety.