This is fitted under a syncro, its in the same location they are in the 2WD's so yes, between the sills..
Leisure tank regs are not as strict as the tanks for engine fuel, so they can be fitted closer to the ground. I think though, due to a syncro being a bit higher than a standard van you would be ok. Chuckle-bus-tom has twin tanks under his syncro fitted by, err, campershak i think is the name. could be worth dropping them a message?..
The bashplate for mine, i thought was a homemade job but i have just recently seen another on german ebay that is the same as mine.
As for the colour, well, i like white and mostly they originally came in white. I also think it will look nice with the blue of the van

Don't really care about it getting dirty, hose pipe will cure that.
Time is a drug. Too much of it kills you