Before stressing yourself out trying to figure out where to even start, I would first back up and clarify some things with your girlfriend.

What type of stone does she want? Does she want a traditional diamond? If so, do not go moisanite. They are different and it shows! Is she okay with a lab grown stone?

How would she like for the ring to look on her finger? Does she want a big ass rock on her finger? Something smaller and more delicate? .7 carats could look very large on some hands and smaller on others. There are places online where you can enter her ring finger size, finger length, and carat size to get a great idea of what size you should order to get your girlfriend's finger looking the way she'd like!

How thick of a band would she like and what type of metal?

The more information you have the easier it becomes.

Also, people will tell you that you don't need to spend that much money to get a nice ring. This is true but I have a bit of a different perspective. I think it would be lovely to get her the nicest possible ring within your budget.