Those are all good suggestions, lizzy; thank you. I'll start asking around.
Meanwhile, if anyone happens to have done this (maybe no one has), experience with what to look for in an editor would also be helpful. Thank you!
I once worked at a writing center that maintained a list of editors-for-hire, so check if your writing center does anything similar. Ours had the name, contact info, areas of specialty, and hourly rate.
In terms of things to ask (from the other side):
*How do they work (if the person is local, can you meet in person or do they work strictly over email? If the latter, is a phone an option -- since it sounds like you want more than copy-editing, some sort of interaction will probably be more helpful than a document full of track changes suggestions)
*What fields are they accustomed to working in? This may matter -- or not. Sometimes it's handy to have someone who knows your field's conventions; but sometimes, you just want the best editor/tutor and a good editor/tutor can work across disciplines.
*Given what you're seeking, you want someone who can explain mechanics and style well while also giving you some room to experiment and get feedback. You want someone who can explain why certain things work better for particular audiences, explanations, etc and someone who can point you toward some models. Explain what you're looking for and see what they say.
If you think it would help to be more specific, you can PM me.