You might want to visit my web site, and get some of your questions answered there first. Perhaps you'll gain a little insight with respect to what is needed. Look under antennas.
There are lots of aspects to operating mobile, and the most important one you didn't mention. That is minimizing ground losses.
You speak of length, but an 8 foot antenna is just barely adequate for 17 meters. It is relatively easy to run a 1/4 wave on 17 which doesn't need a loading coil. Nor will it need impedance matching as adding the ground losses and the input impedance will bring it very close to 50 ohms.
Your home work is cut out for you. Good luck.