I can help here on two aspects, 1 because im currently building in a 20ft storage container, and 2 Im a generator engineer.
containers great but you get alot of condensation in the summer, and when you start adding lights, sockets etc u need to be very careful as the walls are obviously conductive.
with regards to a generator running a welder, its not as simple as curent supply, because a welder is essentially a big transfomer you get alot of current "spikes" you can burn out a generator diodes and rectifiers very quickly running a welder,
to run a 230Amp mig you safely need about 14-18KVA of generator as to cope with the spikes, bear in mind a 3 phase 18KVA generator is only capable of 25Amps per phase in 3 phase or if rewired internally to make single phase output just short of 40amps.
A welder is rated by its output current. To estimate its input power, divide the output rating by 30.
E.g. A 130A welder will have an input requirement of approximately 130/30=4.3kW. A 200A welder will have an input requirement of approximately 200/30=6.7kW.
This is only an estimate; therefore, it is recommended to choose a generator of the next size up. However, bear in mind, that many users will not actually require the full capacity of their welder, a smaller generator would still operate the welder, but would limit the welding current.
To get KW from KVA divide by 0.8 ie 10KVA = 8KW
in short, containers great if you get a 40 footer so you have room for tools etc, dont bother welding off a generator, find a mains supply its alot easier.
Carl
(
www.generator-power.co.uk)