For recommendations on what to pick up next, why not check out the best card games or best two-player board games
Rampage of the Valkyries makes your opponents think twice about destroying your creatures
What players need to know about Dimir is that they have some expensive casting cost cards due to the cipher mechanic and because many of the staple tools have mill or library destruction baked in
Mana Crypt costs zero to cast and immediately generates two mana which recurs every turn, though it has a chance of dealing three damage to you on your upkeep, a downside far outweighed by the absolutely ludicrous upside
It's responsible for many explosive starts, which is why it's often an auto-include, and (presumably) why Frank included it but no additional ramp when doing the math on his Lands count
By design, infect counters are nearly impossible to remove, but Leeches are the way to do it