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You're lucky enough to be riding an Arbor, some of the most formidable "regular" snowboards under the feet of a hardbooter. However, a softboot board is still a softboot board. You'll be able to ride it in the hard stuff for a while, getting used to the new feeling of hardbooting, but the board just wont perform the same as an alpine board under hardboots.
So to answer your question, if you want to dive headfirst into this sport, get the full setup. If you want to adjust, one thing first then the other, get the plates and boots, expiriment a bit, then go for the board. Don't plan on riding your freerider on hardboots for the rest of its days, though! you'll need an alpine board one way or the other.
just my two cents, others may disagree.