Any magic weapon will work fine, as long as it's a monk weapon.

"You can use a d4 in place of the normal damage of your unarmed strike **or monk weapon**. This die changes as you gain monk levels, as shown in the Martial Arts column of the Monk table."
-- Player's Handbook, page 78 (emphasis mine)

That means any simple melee weapon that's not two-handed nor heavy, and also shorstswords.

As for your question on party balance, you and your co-DM will have to observe how much damage and accuracy the monk has in comparison to other characters in the party that are sharing the same role in combat.