Nontypical,
Lots of good input regarding renting/buying. Another thing that needs to be considered as a component of time is how long is your dry season and what are your time constraints during that time? I chose to buy my equipment for several reasons with the most important being that I wanted to do it myself. I also like the convenience of being able to use the equipment any time I want to.

If you look at the money side of it it comes down to time constraints as well. If you rent, you need to keep the machine working as much as possible. Eddie is right, one piece of equipment will not dig a pond very fast. Ideally you need to be able to dig, move/haul, and spread dirt which can't easily be done with one machine unless a very small pond. It would have been smarter do hire my work done but I enjoy the work in progress even though it does get old in the hot summer.

I have been on my projects for 3 years now and will list the larger repair items I have hired out:

Two used scraper tires
Dozer undercarriage (First dozer I bought, I got burned. Second dozer I took a track guy with me)
Dozer steering clutches and brakes
Swing motor seal kit on excavator

I have changed several hydraulic hoses, starter, alternator, changed oil, boiled out radiator, batteries, etc.

As you can see it would have been smarter to hire the work done but that is not how I wanted to work it. It just depends on your goals, time, and your budget.

Craig