Hi Vala,

I can only tell you things that have worked for me personally and may not work for you.

As to being better to buy an established home or an off the plan or a unit. It its more often then not very situational. If you can buy an established house under market value maybe that would be best, or perhaps a developer is looking to free up some capital buy giving discounts off the plan or a distressed seller looking to off-load a unit before the bank does. The only way to answer that is to get out there and find the deals.

Also research as much as you possibly can then research again (its free).

What i use for research and has helped me:

http://www.realestate.com.au
I'm sure you know of this already

http://mygc.onthehouse.com.au/property_research/ 
9 times out of 10 this will have the price at what the owners purchased the house for when you search.

http://www.goldcoast.qld.gov.au/t_standard2.aspx?pid=1095
Gold coast city council's website, which you can search for information about the property, eg. Is the pool council approved, can this block be subdivided without impact assessments from the Local Area Plan, also current and previous land valuations.

http://www.rpdata.com/
This is a service you generally have to pay for but can be most definitely worth it