I totally agree that being able to carry supplies and water is a big plus. Of course, someone using a 400 gallon machine, as the poster is, will never going to be able to carry enough water to do an acre lawn. Hydrants are a great way to go it it is a possibility. Pulling water from a stream or pond is another good way to go. I actually did one job where I pumped water from the guys swimming pool. I did a 17 acre job for a golf course pulling water from a pond and every few minutes we would hear a plop and see a minnow flopping around on the ground. Some municipalities have fill stations and will let you fill your machine inexpensively in minutes. If you are filling with a garden hose at the job site having an auxiliary (nurse) tank can be a big plus. You can carry the water for a second load with you and after that you can be filling the nurse tank while you are mixing and spraying the previous load so when you are ready to fill again you have a lot of the water in the nurse tank that makes refilling fast. Water trucks are another option that sometimes works. I have done some jobs were the customer supplied one. In some of those cases the customer owned a water truck and sent it along.
Of course, there is not always a stream or a pond to fill from. If you are dealing with one large metro area hydrants are great. In my case I have tons of smaller towns and the area I service probably has 30 different water authorities which means I would need 30 different permits, 30 different meters, and 30 different deposits.
Sometimes the best thing is to start with what you can do and grow as the business grows.