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.
"Opportunity is missed by most because it is dressed in overalls and looks like work"- Thomas Edison, businessman/inventor