audiovox aps 25 is cheap as hell and has a two stage shock sensor, if you already have the power locks, it also does keyless entry, if you don't, you will need two standard bosch automotive relays, and two door lock actuators.
I second that, Plus if you get a good brand that you dont have to worry about the brain going out then you can hide it pretty good up in the dash. Several people on the board can take a dash out with ease. IT will slow a theif down unless he has a flat bed. My solution to that one is garage and hollow points.
I received my prestige aps25kb today, looks like a quality system to me, I ordered 2 kits, just in case 10 years from now when they dont make this kit anymore, and the brain goes out, I'll still have a backup.
Installation looks like it will take a little while, no sweat though.