Basic Impetus is an introductory set of rules that familiarizes the player with the concepts and terminology used in Impetus,as well as the game mechanics.
In the full set of rules, Generals have various levels, and corresponding zones of influence. The better the commander, the better your army will respond and hold together. In BI, he is there to familiarize you with his always being attached to one unit, though in the full rules he can be unattached throughout the game.
Countercharges are in the full rules, and are one of the things that a unit can do if placed on "opportunity". In BI, your best bet is good planning, and the use of the "evade" move if your unit(s) posses it.
Missile fire can be quite effective, but that depends upon the weapon. Again, Basic Impetus is an introductory set. I happen to like rolling handfuls of dice.
I like BI as it allows me to build armies of different periods and see how they work. It's also a great game to introduce folks to not just ancients gaming, but historical gaming as well. You can build up two opposing forces easily enough, and then have one to hand for a player to try his hand with and see if he likes the game.
And yeah, the .pdf format is a great idea, and at an excellent price.
Best wishes to you and your new rules!
V/R