Without seeing the size of the dam, the slope of the land or any idea of the distances involved, it's hard to say what the best machine for the job is. The problem with small dozers is they just don't do very well at moving dirt or digging into it. There are also two types of dozers, finish dozers and dozers that move a lot of material. D6 sized machines is where that line really becomes obvious. Smaller machines are easier to get to and from the job, manuver around and give you a nicer finish, but the blade wont hold very much dirt. 150 hp machines on up will have U blades that hold more material and can be extremly effective at doing what you want done. Buying or hiring it out, be sure you get the right machine for the job.
As for getting an excavator, they are great for digging, but terrible at moving or spreading material. They are the type of machine that you get when you have a dozer there to spread your material after you haul it with a dump truck.
Have you considered a full sized loader/backhoe? While the amount of material is much smaller that you can move, with time, it does add up. They sip gas compared to a dozer and they are extremely versitile in what you can do with one. Digging the dam, hauling the dirt, spreading it out is all something that it is perfect for. With water in the pond, you need to dig out a ditch to drain and dry the pond before you can do anything. Dozer isn't very good for this, the backhoe or excavator is.
Another consideration is how deep is the silt in the pond that you want to fill up? Just driving a dozer over it might create a nightmare beyond comprehension. Getting a dozer stuck is something you never EVER want to do!!!
Nothing makes sense financially when you own land, so it's not going to add up no matter how you do the math. Decide on what you can afford, what you want to do, and go into it accepting that it's a money pit and it will cost more then you thought possible.
Good luck,
Eddie