As a ballpark starting figure use around $1800 - 2000 per square metre. This just a ballpark figure to use as as starting point and can easily vary either side. So for $150k for that size dwelling is only around $600 per square metre...... Contact a local designer (the good ones understand budgets and costings) and start from there. Material and product choices will vary the cost to construct greatly so keep that in mind. Building costs are always increasing not going down.

I think directing your question to a carpenter or a builder/contractor might be best.

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

we recently completed renovations on our home built in 1952 with kitchen, laundry and bathroom added in 1981...replacing all the windows cost about $30,000 plus labour, new architraves and painting, we did almost all the demolition ourselves although the legal removal of asbestos wall linings cost about $4000 and we also spent about $1000 taking other rubbish to the tip!.even in a carefully planned renovation there will will always be surprises..one unexpected structural beam cost $900 plus another $1000 to install.....i again suggest that you divide the house into sections and concentrate on completing the kitchen and living areas (with new windows etc) and then move on to the new bathrooms and bedrooms when funds allow