Just one more thing to think about from my perspective. If the cops, sheriff or whoever gets called, and action is taken, if anyone arrested is 18 or over, there is no obligation for police to call parents to advise them of the situation. The only call you may get (if things go wrong), is one of two: 1) your daughter calling to advise you the police are there and there is trouble, or 2) your daughter calling to let you know what her bond is.
It's also not necessarily a matter of trust. It's a matter of an 18 year old female having enough authority and peer influence to control people who do not respect what the wishes are of others. Just because she is now 18, what has magically happened that gives her more power and authority over bad situations with peers than when she was 17? Also, try asking your daughter this: if things start to go bad, what is she going to do? If she asks people to leave, and they DON'T, what will she do? Who would she rather have to play the part of the heavy? Her, or a parent? If she calls you, you are an hour away. Alot can happen in an hour. Is she willing to call the police on people she may know, and some she thought she could trust?
If you say you won't be there, then show up on your own, I think your daughter might be more offended by that, then if you or another parent were there all along.
And, please don't take offense at this - I admit I don't know you or your daughter and could be way off - but I can't help but feel that your daughter is trying to make a statement to her friends and peers that she is "there": she's 18, she's now responsible, she's now officially an adult, and she's eager to show that by having a party with no parents to supervise. This may put a little haze on her judgement about having a parent around.
One more thing, whether a parent is there or not, go by the office for whatever law enforcement agency that handles that area. Find out the officer who will be on duty that night and try to get a meet with him/her and explain the party and your concerns with the neighbor. Provide the phone number for the house, so that if the neighbor starts calling complaining about something, the police might consider calling first, instead of sending someone out right away.
Whatever you decide, may nothing but good things happen.