Honestly, I would rather see you just wait until you could afford a couple more bucks on the install. Luke has hit the nail on the head-mobile installers are just that. Good luck finding them when something goes wrong after they leave, or it falls out of the ceiling, or (again as Luke said) it becomes a missle heading straight for the kiddies in the back seat in an accident-how would you feel about that? Not trying to scare you and I appreciate that you mentioned us, but this is NOT an area to skimp. $100 is about what I would expect a mobile installer to charge. They usually like to make about $100 a day and they are good! Everything is $100 for them it seems. Dunno.
A professional reputable shop will indeed cost a couple more bucks for the install, but for that piece of mind, it would seem worth it to me. That headliner (and god-forbid the roof itself) would be allot more expensive to repair than the few bucks you are saving by going with someone mobile. I would hate to see something happen to you. There are one or two guys here that do side work that you maye be okay with but be careful there also. There are a few horror story threads in this forum relating to these types of installs also!