When buying investment properties there are different levels of passive investment involved.
At one end you have those that will buy an investment property and give it to a real estate agent to manage and don't want to think of it again (apart from watching the rent come in every week). At the other end there are those that will do everything themselves including knocking on the door to collect the rent.
Where is the best place to be - well somewhere in the middle. The most successful property investors treat their investment properties like a business. They handle the overall management of the properties and then have a team taking care of the day-to-day nitty gritty of the properties.
Regarding the brand new or 5 to 10 year old property, you are going to pay a premium for the brand new. A property that is 5 years old will be like new but without the premium. I once bought a unit which was 2 to 3 years old for less than the original buyer bought it at brand new. Also you will still get the majority of the depreciation benefits on a 5 year old property. You also should not expect too much maintenance on a 5 to 10 year old property.
Another option you may want to look at is Defence Housing. They are managed by the Department of Defence and you can be guaranteed rent for 10 years or more, whether they have a tenant in the property or not. They also carry out all the maintenance on the property and restore it to original condition once their contract is over. The pitfall is that you will pay a lot more for the management of these properties (up to 15% or more).
Personally, I would not go for a Defence Housing property as I consider the fees too high and would not agree with some of their terms and conditions. However, considering your emphasis on a passive investment, this may be an option for you.