You don't say exactly what you want the mic for. If its for recording, any decent omni should work - but I don't think decent ones will necessarily come cheap - there isn't so much of a market for them as cardioids. If its for PA/amplification purposes, you should probably forget it; people in the session will be too far away from the mic to be picked up sufficiently to be amplified adequately.

As to picking upsome instruments more than others, its called dynamic balancing. Used it in a BBC studio years ago when all but one of the mics had been taken to an outside bradcast. Have the banjo player sit further away - possibly in the pub down the road - and issue all the bodhran players with Stanley knives as beaters!