The best advice I can give is to look at the state of repairs - what are needed? These old ones are rustbuckets and often need welding repairs. The prices are directly related to the price of steel and new or used the used ones have scrap value.
Old ones are not suitable for storage in variable climates like Cape Town for extended periods- Dry heat with no moisture is best for using these for long term storage as long as the container is shaded otherwise contents will cook.
Get the biggest container you can afford- the longest Cubed (highest) container is easier to resell so I am told.
Standard size used containers come in various states of disrepair and need some extra cash and a blowtorch.- not worth the bother unless you plan on doing conversions.
I found one of your old posts about buying old shipping containers and I wonder if you could help me? I am looking to buy old, out of service containers that don't cost an arm and a leg. I have been looking around and doing some research and I think I'm getting closer to what I need, but any help would be greatly appreciated.