There are plenty of good scanners out there. It really boils down to how many memory channels you want and what type of frequencies your area uses. Most places are going to trunked systems in the 800mhz range. If your area uses trucked systems then you would need a "trunk tracker" scanner made be uniden, bearcat, radioshack.
The antenna also depends on what type of systems you are trying to listen to. Radioshack sells a broadband scanner antenna that works pretty well on most bands.
Powering a scanner is real simple... it only needs 12 volts and normally very little current so it could be wired to a cigarette lighter or just about any switched or un switched 12v source in the vehicle.
Also be aware that many states are outlawing scanners in vehicles unless you are an amatuer radio operator. From what I have found, you're state still allows them but be aware of the laws if you drive from state to state. A quick search on google or any other search engine will show you which states have such laws... just search for "scanner laws"
Check around at some pawn shops. You can normally find good ones there pretty cheap. I picked up a 300 channel radio shack Pro-2050 with "trunk tracker" for $40 a while back.
Radio Shack also sells a book called "Police Call" and "Beyond Police Call" (at least they used to) that lists public service frequencies by city,freq, etc. You can also find frequencies for your area online sometimes or ask a friend that already has a scanner locally.
Good luck and happy scanning.