I think Dc-9's are better than 737-100/-200. They have less fuel consumption and bigger speed than these 737.
The price for buy one DC-9-30 is bigger than to buy a 737-200 but, at the end of the game, we can see a big difference about the fuel costs(yes, they are important in the game).
About Caravelle, it's price is really low but i think at the end it"s lowest speed and higher fuel consumption could make difference, but they are good aircrafts(when I changed my Tu-104V for the Caravelle it was a good deal).