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I think the bikes would be less secure due to the clear glass if you leave the van parked up anywhere? and bit more risky getting changed in the back.
I've had a T5 for the last 8 years, it sounds like you need van with a second row of seats and side window installed as well as insulation/ lining - makes a big difference. Check you can get the bikes in lengthwise (might be worth looking at LWB) and not sure how they would fit across the width.
I think I might get a Caravelle and then take the rear seats out, looks to be a somewhat more car like version of a Transporter, and has car speed limits. Any reason why this is a terrible idea?