Rego is expensive as NSW is stupid. It's 2.5 times the cost of other states but I'm over it. I was considering buying a Victorian car but I don't think it's worth the risk to be driving it around out of state.That is very cheap! I'll keep my fingers crossed it runs well for you. Have only seen one 530i locally myself, not a very common car the E39. Ouch at the cost of rego, most people won't buy a used car with low or expired rego as they won't want to pay a pile of cash ontop.
My next car will probably be a 4x4 with an ARB Bull Bar fitted! Country living!
My only worry would be running a BMW in Australia, any garage bill is LOL $$$ because the labour rate will be huge.
Limited supply of used parts due to people keeping cars forever too meaning sometimes the dealer is the only option.
When I needed an interior Fiesta rear view mirror I was quoted $150 locally. Bought on ebay UK for $20 & P&P.
The garage bill doesn't really worry me as I'll do a lot of the basics myself as well as having the luxury to be able to wait for parts as I won't use it during the week. There are a few good "prestige" breakers around me, and I'll be calling into them to try and source a few of the small things that are missing from the car. I guess it's nice to have the contacts at home and UK to get stuff shipped over if necessary too!