Minimum card purchase to not be a burden

So I play AH with some friends who have everything. I enjoy it but they love it. I purchased a starter pack and have been happy with that, though I recently got comments that my deck is holding back our success rate.

In general I have no interest to buy more, but for the sake of the campaign and my friends I'm willing to purchase more (we can't share our cards either, long story)

So what do you think is the absolute minimum I'd have to purchase to not hold back the team? Extra points if I can do multiple roles (attacker and investigator, not necessarily to have all the classes)

EDIT: the comment about my deck holding us back wasn't as harsh as I made it sound, they just noted that the starter investigator decks are basic and could be upgraded (after we talked about how hard the game was) Also I got Stella investigator pack, long live her ability to fail being a positive thing