Stamp duty and taxes for an established house purchase in WA, you need to allow around $15,000 (price dependent) in the budget for this


That's a fair whack off your profit margin.
The next step one has to accommodate is "This is a business" which most people first/second time around fail to come to grips with. Putting additions into as house renovation based on what "you" would want in a house isn't the best approach and not good if you wish it to be a house you will market when complete.
Not saying all changes additions are wrong, but the majority look back later when calculating profit/loss on a project and see items or changes that didn't actually enhance the on-sale of the renovated property.
These projects are very time consuming and can put a huge strain on relationships, be it family or social network.