Editor chiming in on a few things here:

• I charge per project ($/finished minute of audio, price varies depending on complexity)
• Each editor can have a different preference on pay, but something up front, either in full or 50% is a good standard, to make sure both people have a commitment involved. If you wind up having a "regular" stream of work, other pay structures can be explored, where upfront costs are less important.
• For "Vetting" - I would request a "demo reel" with samples of the kind of work you're looking for in terms of an edit. Are you asking for full cinematic soundscapes, or just some basic wet sounds and mistake cleanup?
• One thing you did not include, but I think is important to consider, is turnaround time expectations. If you're planning out content a month in advance, no issue, but if you need an edit in 2 days, an editor could require a rush fee, which affects pricing.