Used springs are a bad idea.
Always replace both together as a pair, preferably new. Both your springs are the same age with the same wear. If one has broken, it is likely the other will go too, soon.
The rear springs can be different compression rates for different load ratings. The high-roof is likely to have a higher load capacity than a standard van, so unless you can be certain that the donor is the same as yours, you should not use them. Don't think you can "upgrade", as the springs may not be the only thing necessary to increase the load capacity.
Isn't the 2006 a different model than yours? Some listings show a change 2005, so probably no good at all.
Look in the traders section, contact Sturgess and Partsworld, with your full VIN number and get some quotes.