An airport's ARFF index is determined by the wingspan of the longest air carrier aircraft with 5 or more daily departures. Index A is the smallest, with aircraft wingspans up to 90 feet. B is 90' or greater but less than 126', C 126' or greater but less than 159', D is 159' or greater but less than 200', and E is anything greater than 200'.
I *believe* ARFF requirements kick in for air carrier aircraft having 10 seats or greater (although someone correct me if I'm wrong.) For example, Cape Air's 402s I think have 9 seats, so they can fly into airport's without ARFF equipment.
Additionally, an airport is only required to provide ARFF services 15 minutes before an air carrier lands and 15 minutes after it departs. Some small airports that may get something like one daily Beech 1900 flight will simply contract with the local fire department to park a truck on the airfield before the flight gets there and then wait around until 15 minutes after it leaves. Then the truck would go back to it's normal city duty.
As far as truck recommendations, one of the most popular models is probably the Oshkosh Striker, which comes in 1500, 3000, and 4500 models (referring to the amount of water carried on board), although there are plenty more.
If you really want specifics, you can look here
for a detailed list of exactly what type and quantity of agents an airport must have on hand depending on its index.
Many airports will choose to fulfill a higher index than they actually require. This way, if one truck breaks or goes out of service, they have not dropped below their mandated index. If an airport does drop below their required index for whatever reason (trucks breaking I guess would be the most common cause) it must notify air carriers immediately, and if full service hasn't been restored within 48 hours the airport must stop servicing air carriers with an index requirement higher than what it can provide.
Hmmm that may have been more than you really wanted to know....