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.

Probably quite a bit, standard plasterboard sheet is quite inexpensive which is why its used on 99% of homes. Anything different like lining boards, plywood, etc cost way more for labour and materials. Stria is an external product so it manufactured to high standards even when compared to its competitors it is a high quality product. Look at something specifically internal use perhaps, like 'EasyCraft'.

$300k would be the building budget only and will often not include the architect or other professional service fees like surveyors, engineers and certifiers. The biggest point to remember, is the reason behind engagement of an architect. Are you after a great lifestyle outcome in your home. Are you are detailed person wanting to ensure there is a place for everything in your home. Does your site offer something where you can gain by tremendiously. Do you want your own ideas challenged. The fees set by different architects are yes based around scope of service but also based around their skills and so not all architects do charge the same rates. My own service is for the building architecture, interior design and landscape design, you might say what does that have to do with my lounge room extension and in your case there maybe only interior scope or your external spaces maybe also impacted. My services also includes external real costing work from a cost planner, so you'll know how much your project will be very soon along he steps the delivery process. My service includes project management and my services covers off on a huge range of sustainable and passive design principles which are applied to my clients projects for best outcome. This is why architects fees can vary between providers. It is your lifestyle outcome that is at stake, Once you have spent the money you need to live with it or sell so ensure you factor in the gain in your assessment of fees . Be careful even though some architects have alot of positive reviews this does not mean their design ideas and direction are going to be best for you. Just agreeing with a clients thoughts is not helpful to a client. I charge on a combination of square metre and hourly service and percentage only on request by my clients. $3000 per sqm mention above is mid standard not high spec finish . But standard is subjective as well so this must be worked through with your architect.

If you mean a Private Building Inspector, then it is up to you and how confident you are with your contracting building company. The private building inspector will inspect your home at different stages of the build. Slab laid, when walls reach roof height, when roof is installed, when plumbing and electrical is installed and at final completion. If your building company is doing there job correctly then I would only get a building inspector to walk through on your final inspection. But if you feel there are problems with the build, then have a building inspector employed on a regular interval for the different stages of completion. Regarding cost, contact companies for quotes of their hourly rates.