1. Which cable do i need to power it up?
2.5" hard drives get their power down the IDE cable, a separate power cable is not necessary.
2. If i can't get a 2.5" drive can i get a 3.5" to fit in the case?
Some people have managed it! Getting hold of a 2.5" hard drive is easy enough though, since they are standard on all laptops and have been for years.
Steve.