The supplying company will know what you need and should supply necessary spacers in the fittting kit. Don't forget to buy four security wheel bolts!
The standard bolt size on the Mk.II Berlingo is 17mm but although the thread is the same you may find that the new bolts have a 19mm head.
Hellfrauds do a 17/19mm extending wheel brace for about a tenner.
Bearing in mind that the new alloys were only £50 each and similar in price to OEM steel wheel, I don't think its worth the possible hassle of buying second hand from eBay.
I've had Citroen supplied 15" alloys on my mk3 van since new. I found a card in my wallet which gives the following dimensions required for trouble, (and spacer), free fitting of alloys to take the standard Michelin Energy Saver tyres :
PCD (Pitch circle diameter) ie, radius of wheel bolt centres = 108mm
Offset = 15 to 25mm
Bore (centre hole) = 65.1mm
Fittings (whatever they are), wheelbolts I presume = B12 x 1.25
I hope this is some help, but take no responsibilty for errors, as I gleaned this info off wheel dealer websites.