I am not very experienced with the game and have only finished DD once earlier this year.
I recently found a conversion pdf for MM characters and I was eager to play Zadim as one of three characters through DD and Curse Adventure Paths, as I like the Slayer/Assassin archetypes in RPG games.
I just finished the first scenario of DD using Zadim (killing monsters and scouting), Varian (offensive magic) and Drelm (healing and disabling); and even after winning it I found Drelm underwhelming and that Varian was the MVP but on the verge of death by the end of it.
I understand that you kind of need a healer and a disabler for a party, but I don't know if having a spell slinger like Varian is necessary. I was feeling that maybe Kyra and Quinn may be better companions to Zadim if arcane magic is not necessary, but I would rather not underestimate the usefulness of Arcane spells.
Which party members would you recommend for Zadim (max 2 extra characters)?
The most important thing is party balance. Pick characters that look fun to play that preferably have different skills and leave you with a party where any given stat (Strength, Dex, etc.) has at least one member with a decent die size (at least d8 is nice).
Since Zadim's Widom and Charisma are low, having at least one party member with higher dice in those areas would be good. Also, some skills (like Fortitude and Disable) come up a little more often, so those are good complements to any party - the latter can help with traps, obviously. Since Zadim is a good scout (his ability to examine cards and see what's coming), he'll hopefully spot hazards that a particular character is adept at handling (or desirable boons some is best at acquiring), and the party can act accordingly.
You might check out this great blog entry. It's general in nature and not specific to the Mummy's Mask adventure path, the principles are relevant.