Smitty74

If your repeater has a input tone, you need to be transmitting that tone in order to "wake up" (open squelch on) the repeater and use it. If your repeater has an output tone, what that does for you is if you set your radio up to require that tone, your radio will not wake up unless it hears that tone coming from the repeater.  setting your radio up to require the tone is optional, but if there is another repeater using the same frequency anywhere even slightly near to your repeater, you might here static or bits of conversation coming in from far away. I personally find that annoying, and if the repeater offers an output tone, I definitely take advantage of it.

Almost nobody truly cares.

You didn’t give us enough information.  Does the antenna say “ground plane required?”
A ground plane is important for an antenna that is designed to need it.  Not all antennas are. But, that lip mount almost certainly provides a connection to your hood, which will serve as the ground plane.  You won’t damage your equipment.
I would just hook it up and try.

I agree with the others.  The lip mount should give you a decent ground plane.  At these frequencies, electro-magnetic coupling is fine.  Just make sure the screws are tight enough to provide support that the antenna doesn't wave around, fall off or bend the sheet metal, but you don't have to worry about puncturing the paint/etc. to get a good ground.
 
 
 

If the SWR looks good you most likely won't damage your radio. The ground plane will influence the direction of maximum signal. That's usually in the direction of the largest area, metal, around the antenna. With a lip mount the signal strength will be higher towards the side of the car with the hood. How much difference that makes is hard to say. 
If you really want to know just do some tests. Have a buddy a few miles away monitor your signal level on his radio, not the audio, with the signal strength indicator on his radio. Most radios have one, usually a simple bar graph type. Then turn your car so its in a different orientation relative to you buddy's radio, transmit and have them check the signal strength. This would be easy to do in a parking lot. Doing this while driving around introduces too many other variables that affect the signal strength. If there isn't much or any discernible change then don't worry about it.