Some important information to this:
Since we are working at the collosus ,i played a few turn's more "un-officialy" to know when the enemy's would have almost completed the collosus.
And at 225bc i got the message that the english were almost complete with building it. (damn cheaters)
Anyway ,to my calculation ,if we rush buy the caravan at Potopolia emediatly at 250bc ,bring it to Duckpond to add to the city ,then at 225bc we would have enough production and money to buy the wonder ,and the collosus will be ours.