180-190k is way more than Grandis's price. Even if they do return better FC, the amount you saved for choosing Grandis over Estima should be able to cover up FC costs already. But what's with the head unit? You mean the audio/media player? Can't just use back the Japanese one?
For the 3.5 litre V6 variant, roadtax will be same like 350Z which is bloody expensive.
But I think one of the reason for Grandis's high FC is because it is using the old Mitsubishi Sirius (4G6x) series engine? That engine family is very old already. Since the days of old Mirage 4G61 around 20 plus years ago.