You know most that is "unregistered" is a fresh import, therefore, initial rego fee = $1000+
As for turbo vs NA, depends on what you want, if you don't care about the little kick when boost kicks in and just wants a car to get from point A to B, then NA will do the job, but if you like to have a bit of fun sometimes, then turbo is the way to go.
Maintainence - depends on the condition of the car I guess, anything that's in a shit condition will cost you more at the end But I guess an NA will be cheaper to maintain and cheaper on fuel, but not much difference, maybe 1-1.5L per 100km.
I think you should not rush it, look around and wait for a good one to pop up. But again, definitely get it with RWC and rego.