where r u from mate?

as far as i know, if your car didn't come with a left hand mirror than its legal.

im not sure about a 1200 ute, but 120ys could be optioned with 13inch wheels and therefore would be legal to fit 13's.
as long as your wheels don't stick out from your gaurds and your speedo isn't affected by a great deal you should b right.
185/60 sounds about right on a 13 inch rim
the tyres will have to meet or exceed the standard load rating of the 12's

i thought you needed to go to the rta or the local police station to get them to check, not sure otherwise

don't let the buggers push you around just coz u drive a datsun