One quick note, the form your spice is in will have a difference in the amount you use.
The homebrew store's sweet orange peel
is quite different than the preground spice that was used by OHIOSTEVE
You will have to use a different amount to get the same flavor.
In this case, I think it might be worth the few dollars extra to know that you will get the same flavor OHIOSTEVE talks about.
Same thing with the coriander. Ground coriander is what you want to use. Again, this should be available in any market of specialty spice shop. I have heard tell that grinding it right before you use it will give off the freshest flavor. I am not sure how much you would need to add or cut back if you choose to use fresh ground. You can buy coriander seeds and grind them in a clean coffee mill.
The emphasis of this beer is on the interplay between the spices, the sweetness of the barley, the smoothness of the oats, the nuttiness of the wheat and the very mild bitter from the hops to balance the sweet of the malts. It is a fairly delicate beer and any deviation from the recipe will change that subtle balance.
Make it once. See how it turns out in your system and then SLOWLY make changes one at a time to get it closer to the flavors you prefer.
Cheers,
Wayne