New containers are made to a very high specification and have only ever completed one sea journey from China to the UK, which means they have not been consistently exposed to salt water at sea. It is salt sea water that is the main cause of rust in second hand containers. There is very little difference between the price of new and second hand containers but there is a great deal of difference in quality, durability and appearance.
It is very unlikely that you will get condensation in a new container and for that reason we only use new units in our self storage yard. New 20ft containers have 5 vents each side which helps prevent the likelihood of condensation appearing.
Second hand containers have normally completed a good 15 years constantly traveling the world before being sold. Being knocked about on the docks and corroded by salt sea water all take their toll on their appearance and sometimes their structure. However, if the goods you plan to store do not require pristine interiors and you are not too bothered about external appearances then second hand units will do the job.
See the pictures below of the two 20ft units. Both are 15 years old. The first has spent its life travelling the world and the second has just sat in our self storage yard and not endured constant exposure to salt water.