150HP ate or atw?

All turbo engines need engineering for legality in Vic, so which ever turbo engine it wont matter but the A12 would be better on cheap stock gearboxes especially considering you said of only carrying some "IKEA" products in the back from time to time as the heaviest items, while the A15 would require at minimum:
60 series box going up to a T50 dellows or better a Simon "Wild Boar 71" box.

A well setup Turbo E85 fed A12 or bigger bore A12 can be setup by Simon either 150hp ate or atw, EFI would be best with E85 tuned properly by Simon has shown an A12 stocker with its over-engineer internals is all thats needed.
A15 usually have 2mm less piston height "meat" at the top compared to a12 & a14.

For your earlier Grandfather story stated, keeping the original A12 heart aside as a token or memento would be best for respect regardless what you choose to put in.

e85 stations everywhere now
http://www.gps-data-team.com/poi/aust ... nd=e85#Points_of_interest