A mattress in the back and a bit of cooking equipment will make for a very decent simple setup. A few mesh curtains for insect control and a roll out awning on a roof rack would make for a good fancier setup, and for ultimate luxe an airconditioner for a small space like that will probably be within reach too. I’ll probably add a bit at a time over a number of years - starting point will probably be sound deadening and insulation for the walls and roof.