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)
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.