why do you need the ensuite and what do you love about this property? based only on the floorplan i don't believe there's enough space to have an ensuite although, if this is a good house in a great area for a good price the best option would be to add an extension, subject to the usual costs and restrictions but, the ensuite or extension will sacrifice space from the existinhg house and even the example shown outside the house will only work if the side fence is at least 2m away btw your plans and measurements are deceptive because the bedroom next to the bathroom shown as 3.5m x 3.5m is really only about 3.5m x2.7m ... the attached copy of your plan shows some options for adding the smallest possible ensuite (shown as the rounded oblong at 2.5m x 1m) .. an extension allowing enough space to add a master bedroom with ensuite or repurpose existing rooms and add new living rooms with better connection to the garden could be an option but an extension is not cost effective just for a tiny ensuite

My brother is a builder in Melbourne and he says if you allow $21K for an 'average' bathroom, you'll have a decent range of choices without going over the top. My two cents worth on the potential 3rd bedroom - I'd look at some of the wonderful ideas on Houzz for 'disappearing' walls - so you still have the potential to keep the space having an open feel. Search Houzz for "sliding wall dividers" and you'll see what I have in mind eg Re the kitchen - I'd agree on removing the wall to make the space more open plan. But if you don't want to do that, my suggestion would be at least to create an opening in the wall to the lounge eg

Looks like a half day's work to me, so a few hundred dollars including materials. Where are you located?

there are numerous Sydney based building companies that offer a KDRB service and many that offer display homes that can provide an excellent starting point for house plans that can be modified to suit.. check websites that have designs (with prices) identified by width of block, size of house (50 squares=465sqm) number of bedrooms, bathrooms and garages for suitable houses and ask for quotes based on your specific site and be prepared for much more than you expect but probably less than the cost of a house needing major renovations that doesn't meet your needs.. and of course you could hire an architect to achieve a truly custom designed home but a few tips would be to obtain a proper site plan showing exact length of all your boundaries, direction of north, locations of services, a detailed contour diagram showing the precise levels of your land and contact local council to find out the maximum size house that can be built on your block and any restrictions for height or easements