I had some human clippers beore, and they actually handled bridlepaths well (I think the stiff hair was easy on them), faces were ok, but they choked out a lot on leg hair. Oh well, for $15 they worked for me.
Anyway....there is a chance that the horse will be upset by them. You can do some work desensitizing her before you actually clip. Rub them all over her turned off first (she should be ok with this if she's used to normal grooming, but start at the beginning y'know), then turn them on and hold them against her and move them around (just the machine, not the blade...you aren't cutting yet!) so she can get used to the vibrations. Start on non-threatening areas like her shoulder, then move out to the more sensitive stuff like her face.
I've found most of my horses don't mind their chins being done at all though, and it's not a place where clipping mistakes will be so blindingly obvious, it really is a good place for you to start. You don't want to go too short down there, so use little pressure. Never cut firmly against the hair, unless you want a shaved look. If you are trying to blend in clipped to non clipped areas, always go with the hair.
Edit to add - keep clippers clean and oil them as you use them. If hte blades get hot, stop and take a break.