It is important that the excavator has a smooth large bucket. The longest reach you can get is very beneficial, don't get skimpy and use a smaller excavator just because they are cheaper and more easily available.
This is what the bottom of your pond looks like before you clean it out. You will be amazed by the amount of sludge at the bottom of your pond.
These machines are enormous
Here is the excavator in operation
Here is the amount of sludge we got out of a 3/4 acre pond. The best way to get rid of the sludge is to put it over the top of the dike and let it ooze away. Make for sure that none is left higher than your dike. Let it freeze out for 2 years before you try and work it. By then you should be able to level it off or haul it away. Do not let them pile it up alongside your pond and come back and move it later. If they do, it will ooze back into your pond when it rains and you will have accomplished nothing. Have it hauled away and put somewhere else if you can't put it over your dike.
If you ever think you would like to be able to back boats or anything else into your pond, build a ramp out of rock. Large flat rocks on the bottom and two inch rocks with fines covering it up. If your pond is not set up to drain you will need to breach your dike by v-ing it out, draining it, then when repairing the dike, it is very helpful to install a drainage pipe for pond management purposes. Dredging a pond is not cheap, but doing it right is still less expensive than building a new pond.