I brew a wit on a regular basis. I use a wheat extract because it is 65% wheat and 45% base malt. (from Briess via Northwestern). As for the orange and coriander, I crush them on brewday and make tea bags for them. I steep for 20 min after flame out. Many people add them to the boil at 5 - 10 min with great results, either way they should come out of the wort with the hops before fermentation. Summer is a great time to enjoy this style! Cheers.