Hi Gde,
The $165 kit includes a controller, hardware, and panel. It is not expandable.
They offer other kits as well--which come with a 50 amp MPPT. That seems to me to be an 'odd ball' size.
I'm glad solar prices have dropped. Perhaps it is time for me to expand the system and get a 1000 watts (or more) on the roof.
What has held me back in the past is the cost of a large MPPT controller that will function with a 12 volt output. My current mppt works well, but is pretty much maxed out for solar input, so if I add panels it is an "ouch".
There is no question in my mind that new panels are preferable to used ones. On the other hand I have champagne tastes and a beer budget.