First, the add is misleading. Only the inverter is rated for 1800 watts. The panels and charge controller provide a maximum of 300 watts, which would be significantly de-rated for a flat roof-top mounting. With such low input wattage, you're really relying on charging up a battery bank and using the stored power. And the inverter is a modified sine wave, rather than true sine wave, unit, which is less than ideal for most modern electronics (i.e. entertainment systems). Xantrex has a true sine wave inverter configured for mobile applications (Freedow SW series).
I was able to build two setups (600 watts w440ah and 1200 watts w880ah, both 3000w true sine inverter/charger and 60A 3-stage controllers) for less than $5500. IMHO, I would decide what you need for your specific application and buy the appropriate configuration.