Can definitely see the advantage of a smaller block ie. less maintenance etc but the larger block will always be more appealing for the average family and investor especially if it's more than double the size. 300m2 is tiny as though.....my side yard alone is much bigger and situated in the Adelaide Metro just 8km from the CBD. I'm guessing the new home would be smaller than the older one as well. Also if you're talking about an old bungalow or character house that can be restored, it will eventually be worth double the new home......other thought would be to knock it down, build 2 or 3 townhouses, sell 1 or 2 and live in the other if you don't like too much land.
If you were talking about a new home on a 450 - 500m2 block minimum, then that would be your perfect scenario.
Devine are renown for overcharging on squatty little house & land packages then using the FHBG as a tool to sell their little doll houses.
Do your sums, what is a 300m2 block worth in that area??,,,,,add the cost of a 3 br spec home,,,,,,and I'm guessing it'll be less than what Devine want for their packages.
Speak to a developer, residential builder or even a real estate agent if you know one that won't just be looking at you thru $$$ signs.