I think 850 for a a15 sounds about right,but what is sold in Australia as a low k jap motor in Australia was probably a high k thrashed motor in japan.
I had my GX motor fully rebiult for 900 bucks but I removed the motor and cleaned it down and did everything that I could so the engine guy did not have to.
The 700 bucks this guy has quoted you to R & R the motor is an absolute rip off,by my quick calculations he is charging over one hundred dollars an hour.
Why not update your tools at supercheap i.e socket set,set of spanners and hire an engine crane then also buy a w/shop manual a six pack of beer sit back one night and read the manual and give it a try yourself.
The $1620 he is quoting will get you a pretty hot reco'd A12 and you will end up with a pretty good beginners tool kit.
Also let your fingers do the walking phone around your local engine people and get a few quotes.
hope this helps
Ade