replacing orig indirect

Timco

Got a call today about an old indirect (orig asbestos covered giant tank) that is leaking from an unknown source...will see it tomorrow. To size a new tank(s) (36 unit apartment building) do I match up the gallons of the old tank to new indirects, or to new gas water heaters if that route is chosen? Would like to give both prices, as they have already stated the new owners are on a tight budget...

Thanks, Tim