No harm for a fast prime like the 35/1.8 or 50/1.8 if you have the budget. At least it opens up the possibility of shallow DOF shots and natural low light shots if you find that you need it on the field.
remember to bring suffice of battery for your flash unless you have good access to electrical point. So areas have power shortage. depending on where you go.
i would think the following 3 options... but these are just my advise and it may not suit u.
only u can have the final answer...
1) stick to ur 18-105 + use in cam flash. (no $ involve = tell story thru ur pics).
2) stick to ur 18-105 + external flash.
3) 35mm + use in cam flash.
it really depends what u think u shots are...
what do u need and want to shoot...