The varying missions and what should be taken into account can be found out at eve-survival.org.
Basically missions are distinguished in combat, transportation and mining. Different levels can be identified. Consequently you don’t have to be shooting something in order to earn some money. You may accept a transport- or mining appointment instead.
Gul Rashen in his German article provides a comparison of different Caldari ships capable off running level 4 missions. It is very advisable if you do not exactly know yet which kind of those ship types (battle cruiser, commando ship, battle ship, marauder or strategic cruiser (T3)) you should opt for. You might run the article through a translation program and should be able to get the idea behind the ships and their fittings presented therein.