Buy order for new players

First off I would say start with the battle box they are all really solid models that will see play for the whole of your time with Ret. From there it is a lot more open ended. Ret is in a unique spot compared to some other faction as 80% of our models see regular play and 15 of the remaining 20% see play quite often too. There are a few things that everyone should have though.

First Solo: Arcanist
In some cases two or three but some new players would be hesitant to buy multiples of one model but at least one of these guys is a must. They show up in every list and I have yet to see a case where you are not using at least one.

First Unit: Mage Hunter Strike Force/Sentinels
Sentinels are probable the more competitive choice here but I have found that a competitive focus comes with playing the game for a long time and that new players are more focused on having fun. Strike Force really fits into the theme of what Retribution really is and they still see tons of play and are by far our premier ranged unit.

First Warjack: Shyeel Heavy Kit
I know the Battle Box comes with 3 but Retribution has really solid jacks and you will see a lot of heavies especially the ones on the Shyeel Chassis. I would strongly suggest magnetizing it but I know that can be scary for players who are new to the game.

First Non-Battle Box Caster: Kaelyssa/Vyros1
Magister Helynna is a solid War-Caster but some of the most cost effect way to add a whole lot of variety to a new players collection is by picking up a second caster. My personal vote is for Kaelyssa with an honorable mention to Vyros1. Kaelyssa in a sense is what the retribution is and she can take almost anything and make it really shine. Vyros1 gets a honorable mention because he is super simple to pick up and play but he plays very similarly to Helynna so my not provide a new player the variety of styles they are looking for.

Other Mentions:
As for the longer list of other models to consider really anything falls into here. When I started playing the game I bought what ever I liked the look of no matter how good or bad they were and I think that original thirst for fun is what has kept me in the game. So I would suggest some models that have particularly interesting rules that just add tons of variety to your play. Elara1 is our journeyman caster and those are alway really fun to play with, Mage Hunter Assassins because for a new player there is nothing more exciting then spiking and one shotting a light jack, and lastly any form of Eiryss, you may want her third form if you are running strike force but if you are going a different route either of Eiryss1 or Eiryss2 can be a big bundle of fun rules and a real one person wrecking crew that can be really satisfying for new players.

The MK2 battle box is a great buy for new plays since it gets you Kaelyssa as well as more jacks.

I'd say the first buy after the Battlebox is without a doubt 2 Arcanists. It is like buying a Choir or Paingivers. These are basically the models you will use everywhere. From there, I can see the argument for Sentinels. After that, it all depends on the list the player wants to build up.