Why are they cheaper?
Generally when you buy a property, you are in fact buying two things. You are buying a structure and the land that it sits on. Over the long run, the land is what really does all the heavy lifting in terms of gains in the value. When buying an apartment most of your money is going towards the structure. With an old house it goes mainly towards the land.
Body Corporate
If you are looking at apartments, learn about body corporate. It is how maintenance and other issues with the building is managed collectively with other owners. For existing apartments it is prudent to go through the past notes of meetings of the body corporate. Here you will uncover any upcoming or past maintenance issues. Also the rules of a body corporate may have an impact on how to choose to make any changes, for example some do not allow for outside additions such as heat pumps.
Off the plans
Buying off the plans or an apartment not yet constructed can carry its own risks. It is important to understand the expiry for settlement dates if your bank funding approval only lasts for a few months. Older apartments will have a history of maintenance which give you a peek at the quality of the build.