Initially, it's easy to fall into the trap of buying smaller capacity based on "current" thoughts on how much the unit will be used. Unless you are buying for a special use, I would go with at least a 200 gal unit. Your business will grow into it more than likely. The parts that cost you the most on a rig are the pump motor and hose reel...not necessarily the tank. Conversion of the unit later to more capacity or even having to buy a "bigger/better" unit can be a headache not to mention even more money spent.
I know your plan is to spray low volume with a handheld boom but even at that you will be spraying at the 1/2 to 1 gal rate. If you are serious about growing apps business you should easily spray 150 ft sq per day at some point soon in your career. For this reason, I recommend at least the 200 gal unit as a starter. JMO
There is a 200 gal unitin the Equipment forum
"The Poor Fish" circa 1930's: The Poor Fish wouldn't have been caught if he'd known enough to keep his fool mouth shut.