When purchasing a unit for investment there are many things to take into consideration.
Always choose a small block no more than 18, and make sure you try your best to get one with a balcony as this allows your tenants to have an outlook and, most of all, some fresh air.
Garaging is very important and definitely helps to achieve a higher rental and a lower vacancy rate.
It is also very wise to choose a unit block that has character, something that can't be repeated easily. So go for something that has high ceilings throughout, and loads of character from the streetscape. The units I am talking about are usually dated around the early 1900's and have been built to last.
There will always be one bedroom bigger than the other in a two bedroom unit, but make sure when choosing your investment unit that you try and get them as close together in size as possible.
What you don't want is too much competition, so be sure to purchase your investment unit in an area where there are not too many new units being built otherwise you will struggle with your tenant to put the rent up each year, simply because they will have a lot more to choose from.