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There are some difficulties purchasing modules a few-at-a-time as you can afford them. The worst is that module design is constantly changing, so the panels you purchase a few of today may not even exist next year. My array of 18 Seimens SP75/ProCharger 4 modules are years out of date. If one failed, I'd be sent a warranty replacement that was nothing like the original, wouldn't fit my racking.
The good news is that dissimilar modules play nice together when connected as an array. The caveat is that every module has to have the same number of cells, usually 36 for a 12 volt panel. Avoid mixing 33 and 36 cell modules in the same array. You can also find high voltage modules with 42 - 44 cells, these may not be mixed with the 36 cell modules, either.
If you can be flexible in your racking (build as you purchase modules), you'll probably be fine.
All of this assumes that you are installing a 12 or 24 volt system. If you are looking at a high voltage array to feed a batteryless inverter, module matching is a much bigger deal.
Remember that modern MMPT charge controllers typically have a higher input voltage than the battery nominal voltage. My 24 volt system uses three of the Seimens SP75's in series for a nominal input voltage of 36 volts (usually 48 - 52 volts when the charger is operating). the charge controller bucks the higher voltage down to charge the 24 volt system. In a situation like this, you'd have to purchase the modules 3 at a time, and they'd have to be closely matched, same make/model/wattage.
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