Hi there,
I would seriously advise against using the libraries. Obviously, I'm a composer and so I am biased, however, hear me out.
Yes, you get some pretty good sounding music in the libraries but what you will not get is input from a person that feels music. I'm not suggesting that you don't!!!! But it is always a little bit different for anyone.
Second: pacing, the pacing of your film is very very personal and so should the music be and follow the pacing of your film. If you use ready-to-go music you'll be cutting your film to the music and thereby have a totally different result from what you wanted to begin with.
Third: The process of listening to someone elses ideas is crucial for any filmmaker starting out as it will help you focus on your film from a completely different perspective and open up angels that you didn't think of before. Don't deny yourself this experience.
I agree having a composer work on your film is the first time you will somewhat lose control of what the film does, but this is normal and you should learn to deal with it creatively.
You will also build up vocabulary so the next time you speak to a composer will be much more efficient than the first.
Good luck and best
Hans
PS: yes, I'm plugging my stuff now too:
http://www.hanshafner.de