first make a plan of the whole block and identify areas for different purposes, just like rooms inside there will be different areas that will suit different purpose and can be prepared in different ways...eg a clothesline area, driveway, kids play area, vege garden barbq area, future pool, spaces for shade or screening from neighbouring building or wind.... even if you're not able to add final touches this is the best chance to improve the soil ...have the bob cat remove the topsoil and level the underlying surface and then you can add some clay breaker and blood and bone to the topsoil for the bob cat to mix in and replace over the levelled ground ...you may even decide that some mounds could add interest to the area but mounds should be made out of out of piles of rock or rubble for better drainage and then covered with soil..then plant all the exposed earth with grass seed and encourage a lawn while you rest and save up to start adding trees, garden beds etc...the lawn will look clean and neat and you can dig up small sections of lawn at a time (and turn that into compost) when you're ready to make a garden, enjoy

Please be sure you locate where all service lines are and know the depth at which they are buried before any work is carried out. I recommend selecting a see based on you right particular area and needs. Consider amount of sunlight, shading that exists, number, size and activity level of dogs, how much watering is doable and allowable, and other maintenance needed.

Sorry, I meant select seed based on your particular...

Some reading:

https://lawnsolutionsaustralia.com.au/grass-types/

If the budget permits, it may be worth spending some money on a landscape designer to come out and give you some suggestions and advice or even do the design for you. You can find designers local to you through professional bodies like Landscaping Victoria or AILDM. Most designers will actually have a consultation service. Even though it may cost a little now, it will save spending hundreds on something that doesn't work. Often AILDM or Landscaping Victoria will run Design Clinics at horticultural shows and events where you can sit down with a designer and get free advice.

If you're wondering what to plant, talk to your local nursery as they will be able to advise what works best in your area. Just make sure you follow through with a purchase - no-one likes to be taken advantage of! You can also check out what the neighbours are growing as this will indicate plants that work with your soil and climate.

A tip when designing for plants - if you want to get a layered effect in your gardens, make sure you have decent width garden beds. Where you can, go for beds at least 1.2 or more wide. I keep having clients that want that look but leave me with beds that are too narrow to achieve it!

When it comes to installation, unless you're really good at handyman type stuff, get a professional in. It may be that you do things in stages to work with your budget, but again, this will save you money as it will be done properly and will last.

In response to your specific question, if you have pets, I'd spend the extra money and look at getting instant turf. Getting a warm season self repairing grass like buffalo or kikuyu established from seed at this time of year is almost impossible. They have a long germination time (often up to 6 weeks), and need warm weather to germinate and grow. Even cool season grasses like fescue take time to germinate, and then time to get established enough to allow for regular foot traffic.

To improve heavy clay, adding gypsum will help in the shorter term, but longer term, adding organic matter will be the best solution. Lots of compost, and mulch with organic material that will break down and add to the soil. This is better than adding a layer of topsoil, though in areas where you are going to put lawn, a lawn mix to a depth of 150mm is worth it. Make friends with your local arborist for cheap mulch.

Don't be scared to plant smaller plants. While they don't provide instant impact, they will establish quicker, and often outstrip their more advanced counterparts in a short space of time.

Good Luck!