Season 1 was SR3. That said, they aren't that hard to bring up to SR4 just using the grunts out of the main book, they were fairly straight forward missions and easy to boot.
Quickness was the SR3 equivalent of Agility. Reaction was a derived score that was based off of Quickness and Intelligence (the parent of Logic and Intuition). Initiative was a base of 1D6+Reaction with Initiative Boosters (Wired Reflexes, Synaptic Accelerator, Boosted Reflexes, ect.) all giving +1D6 and +X Reaction. Instead of having IPs, character rolled initiative, totaled the dice, added reaction and got a result (for example, rolling 5 on a D6 and adding in another 6 from reaction would be an 11 Initiative). Every one went in order of initiative, then the GM would subtract 10 from every Initiative score and anyone with 1 or more would go again, rinse and repeat until no one had a positive score. Then the new round began.
Season 2 is likewise, very very easy for beginners.
As far as Intro runs go, don't worry too much about focusing on what the character's role is and focus more on the character's background. The players will find a way to apply their role to the situation. Players are crafty that way.
If it's a matter of just getting them used to the rules, hand them each an archetype and run the Food Fight or A Night on the Town (Free RPG Day 2012 supplement) and run that as a primer. It will get them to understand the basics, and get a feel for the character type they want to play.
