Operational Electrical Panel in an unfinished building -- safe? Sane? Legal?
If your solar system is large enough to provide power it is legal, sane, and safe. For temporary power all the circuits will need to have GFCI'S, if the panel is exposed to wet conditions you will need a nema 3r type of enclosure (outdoor as you mentioned) the last if not under cover you will not be able to use NMB or Romex wiring it will need to be in conduit and use thhn/ thwn wire. With weather proof boxes , and in use covers. I have seen temporary power using standard breakers and WR rated GFCI outlets this may be a better way to go because the circuit breakers don't have the electronics sealed (I have had to replace GFCI'S in a nema 3r panel in just the last year on one of my temporary power setups). 

In my area a temporary power permit is required. If you are only connecting to your own solar system I don't know if I would get a temporary permit, the temporary permit provides an inspection and a green tag so the power company can connect to the panel. If you need more power than your solar can supply then a permit / inspection would be required. In my mind your solar would be the same as running a generator for temporary power and no permit is needed for that.