I used to work at a jewelers. A good quality moissanite is gonna be as close to an actual diamond you can get without getting an actual diamond if that’s what you’re looking for.
As far as metals, gold is the traditional choice but sterling is both less expensive and more durable. If you do go with gold, I would opt for 14 kt over 18 kt personally. The higher the karat number, the purer the gold, which sounds like a good thing but from a practical standpoint is a bit of a PITA because this in effect means higher karats are both more expensive and more fragile because gold is soft. Since typically an engagement ring is worn every day, something that can handle some wear and tear is a plus.
For settings, look for something fairly low with prongs that have a decent heft to them. Tall settings bump into shit ALL THE TIME and eventually the prongs tens to shift and the stone goes flying (again, gold is soft).
Also, look at her other jewelry and talk to her about what her tastes are. There’s no rule that says it “has to be” anything if a more standard ring isn’t her thing. Going with her favorite color or her birthstone and working from there can be a good option.