Prospective marriage visa (subclass 300) for Australia

Hi I am Farabi. I am Australian Permanent resident and on my way to be a citizen.
I am from Bangladesh. I am in a relationship with someone from my home country for around 1.5 years.
Iam thinking to bring her over here on prospective marriage visa (subclass 300)
Point to be noted, We never lived together and we are not engaged.
However,  We have hips of text messages and call history.
Anyone can answer me the following questions from their experience ?
1. How strong is the possibility to get the visa?
2. How much time it can take for the visa to be granted considering she will be applying from Bangladesh?

Do you need to have a GED or High school diploma to apply for subclass 300 visa?