18 house site for $115,000 prime loc WA

Ive found a great block, between Mandurah and Bunbury in WA, for sale for $115,000(private elderly sellers)
It is 8800m2 and zoned R20 Residential.  The site has very long frontage to a kerbed sealed road, and is over the road from a medical centre, and 200m to shops, pub, PO and rail to Perth/Bunbury.  As there is no deep sewerage in place, I can sub-divide this into 4 x 2000m2 lots, but I want to divide it into 2 x 2000m2 lots and 1 x 4800m2 block.  The current owners also have planning advice to suggest this block can be divided into 9 x 1000m2 lots, which can then be divided in half again(18 house site), as the deep sewerage wont be connected for 5 years.
Now Ive bought in this town before, got planning approval for a similar block to be divided into 3, then sold making 50% profiit in 1 year.
The difference this time is I cant just buy the block, as Ive just purchased a PPOR, and cant really afford it right now.
I have however, spoken to the owner who is willing to accept a $5000 non refundable deposit, in return for settlement in 9 months.
My plan is to secure the block for $5000, then get surveying + WAPC conditional approval for subdivision done asap(last time it cost $4000 and took 3 months), then onsell the whole thing with approvals in place, or sell 2 x 2000m2 lots and keep the 4800m2 site.
Im in the Real Estate industry, and my due diligance values the 3 lots for at $200,000(very conservativley), which minus the deposit and approvals cost ($10,000) is a $75,000 net profit.
Question is…what is the best way to go about this to maximise profit, and hopefully, not pay Stamp duty etc.
The other thing to remember is Ive sold another IP this year which will yeild me $200,000approx(300%)…so I may be able to offset and costs for this development against the profit of the IP…correct???
I dont want to do an option as I think that will scare them off.

Would love to hear anybodys thoughts, particularly if you can recommend someone who has experience in this area.
TIA