I'd opt for not too tough designs with more firepower than sense 'Mechs, in medium to heavy range with not too much speed. This would teach tactical importance of cover and partial covert to mitigate damage yet limited speed of medium to heavy designs means they're relatively easy to hit. Thin armor and a lot of firepower means quick deaths. A lot of firepower highlights importance of heat management, and hopefully thin armor makes pushing heat high attractive.
Pity the intro box has no Marauder and Warhammer, they'd be quite ideal here.
Catapult C1, Orion V (the one that trades armor for another SRM), JagerMech (LRM variant offers a bit more firepower), Zeus (perhaps the PPC variant), Cyclops all have thin armor but relatively high firepower with middling speed. For something faster, the Dragon and Quickdraw are reasonable as they have weak armor (Grand Dragon for extra firepower might be good but it is considerably more valuable than the Quickdraw). Throw in the Enforcer and Vindicator or perhaps Trebuchet and Dervish.
I get the idea of showcasing speed and extreme durability along with other stuff, but i'm not sure they're particularly enjoyable at first. Speed may be difficult to utilize for a newbie, but also difficult to deal with for a newbie. Awesome etc. just tend to become turrets, which isn't particularly interesting.
For starter duel, the Enforcer vs Vindicator is good definitively, though i think the Enforcer has slight advantage as once it is at medium range, it has higher damage output, the Vindy needs to keep its distance to stay on top of the fight.