The bits marked in grey are usually spare pieces, most often as a result of the sprue coming from a similar kit with a few redundant pieces still attached. Keep them as spares, you might find a use for them on future kits.
Sorry, I can't help with the spray question as I'm fairly new to this hobby myself. :o
If your 350ZX is the Tamiya kit then you'll have fun, everything fits together really well and it's a really good looking car.
