Unless the person who is installing your alarm is very good with his work, I would suggest shopping around for the alarm. You want to make sure it gets installed correctly and well. And with shopping around you can make sure you get all the options you want.

As for a $100 amp, you are pretty limited...but it would be much better than powering the amp off of the headunit...I am actually surprised you can do that. What model sub do you have?