My advice is if your money is limited and speed is your primary concern, spend the money on a pointy hat, a decent skinsuit and some clip on tri bars. For the same money as a cheapo, slow TT bike, you will get a very good helmet and skinsuit, which will provide a far greater time saving. You can bodge your road bike into a decent position most likely and given you have two consecutive days of event you don't want to be swapping much componentry between bikes.