Try to find out the various frequencies in use by: City PD, Sheriff, State Police, Fire Departments, Ambulance, Rescue, Civil Defense, Street, county road, and hiway maintenance(good for blizzards), game wardens, hospitals, med choppers. Try to find out the paging freqs. too in order to hear them do the dispatching.
A large part of the time you'll find ancillary frequencies, detectives, swat, narc units, dog catcher, school bus, etc., that are close by the regular frequencies so they only have to carry one radio or not have to have separate radios at the base for every frequency.
If you can find a little old lady that listens, she'll probably know them by heart. Another good source is someone who works for the radio company that maintains all the radios.
Most of the new ones should cover all the bands up to 800/900 uhf frequencies.
When you have a guesstimate on the no. of freq., you'll have a good idea how many channels you'll need as well as the frequency spreads. Then if the no. is close to max on 1 scanner, you can step up to the next one for some leeway.
A lot of the bells and whistles never get used and as was said study the manuals to see if you want anything fancy.
The portable ones have reception that is basically just as good as the base and mobile units. With the portables, you've got a working unit if the power goes out. With the jacks for power and charging, you can easily rig some batteries to have as a backup supply.
If you run into reception problems or a salesman tries to bump you to a better unit for better reception, then you look at external antennas for the house and/or the car. It's gonna be the antenna that will make the difference.