My mini solar system.

In answer to the charge controller question.  If you get more solar panels then your present charge controller can handle - just get another charge controller.

Getting more batteries might mean you will want to get more solar panels, as a "general rule of thumb" you should have 10-20% of solar for battery power.  So if you have a 120 amp hour battery then you should have about 12 amp panel set.  Think about it - you use half the battery (50% Depth of Discharge = DOD) 60 amp hours and then charge it at 12 amps - you need 5 hours (5hr x 12a = 60ah) plus 1 hour because battery is 80% efficient at taking a charge.

Battery life is shortened by deep discharge and by being left in a discharged state.  Faster charging is better for batteries because there is less "permanent" sulfating.  Get a desulphator for your batteries.  Your batteries are your weakest link - do the most for them.  You might also want a battery charger (grid plug in) to compliment the solar panels on cloudy/ winter days.

If you actually do get a "battery meter" (Trimetric) then it will upgrade with your system no matter how big it gets.  The battery is the only component that won't upgrade however so get a set big enough to only discharge 20-30% for your power needs (ex: 60 amp hours x 12 volts = 720 Watt hours so a microwave that takes 700 Watts will run for 1 hour).

For a cheap meter "system" get a "Watts Up" or "Doc Wattson" (ea. $60 at www.powerwerx.com) for the charge controller to battery counting (Watts).  Get a "Kill a Watt" (ea. $33 at www.altenergystore.com) for the inverter.  Then - make sure that you always have 20% more Watts coming in then going out.