At the risk of everybody rearing up at me, I would suggest a hamstick as a starter.
I started with hamsticks and had fun. Now I use a Bugcatcher, but I keep the hamsticks in reserve.... A hamstick will be cheap, efficient enough to make contacts on 40, and will allow you to see how much you really like mobile HF. Hamsticks for 20, 17, etc will not cost that much more, can easily be stored in a Highlander, and will give you capabilities on bands where they are more efficient.
As G8YMW says, bonding is very important, and K0BG should be your bible.
And in the spirit of K0BG: drill, baby, drill. A Hamstick properly chassis mounted will outperform ANYTHING on a truck lip or mag mount. And don't be silly about resale value: when the time comes, you take off the ball mount, put in a round-head bolt of the right size, paint as desired, and fuggedaboutit!
I LOVE HF mobile - and i hope you have fun, too.