Congratulations! Are you nervous!?!?! It's a wonderful feeling getting this far....
Couple of suggestions:
While I don't have a Ph.D in mounting hdd's, I have had 2 mounted in computers, each with well over 1 year of use with no problems. Mount it vertically - screwed directly to the case. Of course, I've used full-sized hdd's. Notebook drives are more rugged, but if a full sized drive survies years mounted vertically, a notebook would last all that much longer.
Boot it up BEFORE you install it!!! Lay everything out on your kitchen table and plug the sucker in. Put it through the paces. Pull the batt from your car if you have to. You need to verify that things work before you start working with power tools in close proximity to computer components!
Don't trust velcro. If that's what will be keeping it from taking a 3 foot dive to the road below, don't use it. If the computer could go flying across the car if you hit a pothole, don't use it. Bolts or bungee cords work much better! And I haven't had a problem with vibrations, potholes, tree stumps, mud holes, snow banks, -15f, +90f (+100f caused lockups), or other such things.
If your wheels are severely out of alignment and causing massive vibration, you may want to get that looked at, though.
If you add a CD-Rom, that would certainly be more susseptable to vibrations - I haven't put mine through the paces yet.