Sounds similar to what I had - 30 foot farm pond that had filled in over the years so it was only 3-4 feet deep. I hired an excavator for $100/ho with operator. Took him 6 hours to enlarge and deepen to 10 foot depth about 45 feet across round. If you can't go much larger you could always go deeper, especially if you have a use for the spoils elsewhere on your property or can use them to grade the perimiter of the pond.