Yes, this is very much a 'how long is a piece of string' question! And the answer is probably very much a 'you get what you pay for' answer! We have just built a house and paid in the vicinity 0f $1800/m2 - the builder we used has 3 lots of 'specs' for houses - basically the lower budget, the middle, and the higher end. For our given floor plan they could quote on each of the specs and we went with the middle one. This involve things like the quality of the fittings/appliances etc. eg. with the low spec we would have got a basic kitchen with formica benchtops, the middle spec included soft-close drawers and hinges with stone benchtops etc and it was the same for all the fittings throughout the house - toilets/vanities/laundry fit out/quality of carpet/type of concrete for drive etc. A word of warning - we had a friend who was very happy because he built a house for just over $1000/m2 - but you could tell! It looked all fine when it was brand new but after a few months you could tell the poor quality of some fittings. And the carpets were already looking worn! so - you get what you pay for.

As C R says, you will need to check with council or relative authority before planting into one of their assets. There's a lot of subsurface assets in a bio retention systems (drainage not to mention the soil itself) which they may not be happy about you disrupting. They will also be able to advise of the preferred species for your area and taking into account the filtration media depth (you need min 800mm media depth for tree planting in a bio swale or retention system).

We have a similar set up to you. I don't recommend seed as we got so many weeds come through. We cleared that and turfed it and love it. Lots of hard work and upkeep but worth it. We have done hedging along fence line and the half acre of grass and the rest we have kept the natural bush. Eventually we will be making it more of a useable bush area with grass surrounding the gums.

anything's possible and you could potentially start a new trend with the linear drain through the centre ..here are the positions that i'm aware of (circle shows the actual drain and stripes the linear grate in alternative directions but either of the third options could be quite interesting as long as the tiler can create enough slope for it to drain properly...look forward to seeing what you decide