When I got my X10 from amazon, i never changed my unlimited BB plan and kept it for the 29 days I had the phone. I actually found that easier, because Amazon just sent you a phone with a sim, it was up to you to contact At&t to switch the devices. At&t asked me if I wanted to switch or keep my plan.
During that 29 days, I used 10Mg of data, I found because of my conditions of working from home, I stay on Wifi and don't incur any real charges, so when I got my Streak at Bestbuy, they asked again if I wanted to keep my unlimited plan. This time I declined and went with the $15/200mg plan. My first month I used 20mg.
YMMV
If you are very mobile and use google maps directions, or allot of apps that are data heavy in 3G you could easily exceed the limits. I used 15mg of my 20mg in 30 minutes on a drive in Tampa using Google Maps to direction us to a location we hadn't been to before.
I doubt where you buy it matters, much, except when I picked my Streak up at Bestbuy they insisted on getting you into the system and onto their dime, when I got the device from Amazon, I took a week before I called At&t and switched my service. Before then I used my old sim from my BB and had no issues. You should be able to do the same with your iphone sim card.
Personally, I wish Amazon was selling the Streak back in December, there experience of purchase and return was far superior to BestBuy. I have also bought Blackberrys from Wirefly without any issues.
Last, you might also want to look at BestBuy's recent return policy changes, looks like it might be a good deal.