Q400 & S2000 are rather efficient in those distances, as they are quick turboprops. They can do 2 850NM travels outside the 00h00 - 06h00 zone(850NM is too small for a profitable night travel).
Or you can go jet for A158s, SSJ-100-95, CRJs, Embraer Ejets, COMAC ARJ, Mitsubishi MRJs that kind of jets. A318 or B767-600 are not that big, but are one class bigger than its regional jets counter-part. It can be a wise move if you plane to later need longer ranges, and you'll already have the fleet group.
But if you limit yourself to your initlal zone, then it's clearly overkill. You'll spare money going for any of those excellent regional jets - or even turboprops. The salary differrence between medium & large aircraft, plus the bigger crew required, plus costlier landing fees, plus..... make the A318/B736 a wrong choice in your case. They are designed for bigger lines & longer ranges. Even if demande goes up, better add a plane than going for bigger planes on such lines.
I've got 3 fleet types in my current company in GW3 : A148, B737ng and 777. A148 fly up to 2200NM, B737 up o 4000NM, and 777 to the end of the world. I try always to use the smallest plane possible on the line. I can make exceptions, like for very busy routes, like Glasgow Heathrow(4 737 in the middle of plenty of other craft) or Nantes Algiers(4 737, with one single opposing flight), but A brand new A148(70 seater) costs 16M, a 737(130 seater) 100M, and a 777(350seater) 490M currently. So I keep the 777 for the very long lines(currently just one, Glasgow Sydney, but I've got plans for others).
in general, going for such a small line with an aircraft at full capacity is an open door to opponents attacks. I'd take a S2000 or a BAE ATP. Q400 is nearly already too big for 100 pax lines. You'll use one, or two. If you use two, they'll be full only at a limited price, and if you use only one, he'll be gushing cash...until opposition mysteriously appears & demolishes your profit base.