Your existing 2013 model doesn't have this system to consider. Less to go wrong is not a bad thing. The residue is most likely just spilled urea near the filling point, but you need to know the fault codes to determine why the countdown has started. If there are crusty white deposits along the exhaust, the injector may be leaking and that's a fraction of the cost of a urea reservoir. If it turns out to require a urea reservoir it can't hurt for the original owner (vs bought yesterday owner) to ask Citroen directly if there is any support available. It would have to go to a dealer for diagnosis if asking for support. Or you discount the price accordingly for the unknown and fix it yourself.looking at a cheap one owner '15 with impeccable service history & reasonable miles - but !
white residue in boot and countdown message makes me wonder if it isn’t more serious than the bloke says.
Out of warranty last July, could it be worth the vendor or I approaching Citroen - or just walk away from buying trouble?