You must use their expansion battery packs (I bought the Delta Pro + Delta Pro Battery so I have 7.2 kWh). Hopefully they'll continue to make newer packs backwards compatible for more future capacity upgrades.
You can use any solar panels you want as long as you don't exceed 150V PV input.
I'm deciding if I should chance it with 440W panels where Voc is 49.5V at 25C (and thus 3 in series will be over 150V at lower temps), or stick with ~370W panels that are around 45V Voc so at the lower temps my area sees (down to mid/high 30Fs around 4-7am) it'll still be under 150V. I'll likely play it safe. Only considering the 440W panels (pretty much any 400W+ panels is about 49.5V Voc) because Will uses them and only calculates their STC Voltage (though his SCC can likely handle more so there's overhead), but I never hear them talk about the Voc when temps drop for him. Most of the times, my temps are well above 50F, but there's times in winter where it gets cold (but no sun out yet really).
My entire house right now runs off 50A, which at 120V is 6kW. Most of that is the AC. So 3.6kW 120V output should run most of my house. I'm planning to run virtually my entire kitchen and my freezer, along with all the PCs in the house.