It can be a problem if one of them overshadows the rest and the DM has trouble calibrating encounters.
For a monk flavored item I'd suggest something that adds a fixed amount of ki, maybe 1 to 3. This will let the player play more with his monk abilities, and wont be overpowered at higher level when damage starts catching up to other classes.
The type and rarity of item really depends on the prevalence of magic items in the campaign. But for low levels its much safer to keep items on the weaker side. But depends on what the other PCs are getting.
Items that include non-combat utility can be more interesting, but depends how much the campaign is combat-focused.