First safety tip of the day is be aware that the metal is heavy and your unlikely to know how heavy unless you have either lifted a precut piece or have just cut it out and now realize its heavier than you thought. Would advise when cutting to make sure you organize things so it falls the right way as well as make sure nobody can wander in the way. When moving parts its worth getting another pair of hands to help rather than struggle.
When cutting always try to cut from the outside either by using a plasma cutter or as I have seen several people use an angle grinder. This allows the fumes to escape rather than build up which is what would happen if you worked inside. Would also advise wearing a mask as you don’t know the content of the paint and removal of floor prior to cutting. If using a disc cutter of any kind make sure your working in an area that doesn’t have passers by due to the risk of sparking as well as broken parts of disc flying off. Always wear eye protection.
Plasma cutter will rank above using a disc cutter because of its ease of cutting once setup.Adding metal tubes etc as straight edges as a guide will glide you through the motion of cutting the steel. The time saved especially if working in extreme hot or cold conditions is phenomenal over using a disc cutter but if you have the time and a tight budget would go with the disc cutter unless you can get a rental of a plasma cutter. Bit of an expensive tool to buy for a one off job. Remove the floor in advance of cutting its generally full of pesticides and of no use in a container home, check you have everything it needs as some require a combination of compressor as well as a specialist outlet all can be hired or bought but best to ask while at the store. Be aware there is a potential of fire so be cautious of your surroundings no point cutting on some dried grass on a field for example unless you fancy making the evenings TV report. When your cutting get on with it the tips will burn quickly and are expensive so utilize them as much as possible. You will get hot pieces fall away and this is why you need to make sure there is no hazard that could start a fire from the heat. One of the most important things as a final note is always work in pairs, you could be cutting and not see the potential of a fire breaking out behind you as pieces drop concentrating on the job at hand, you are at risk of falling steel plate and at the same time everything such as measuring is easier with two as your doing a lot of vertical work so if clamping things down for a straight edge to cut having a guy on the top while your lining up the bottom means you end up clamping things a lot easier saving you time and making sure the measurements are correct. Measure twice cut once is the golden rule.