I'd take the cheapest base model (1.6GHz with Intel 6000 graphics) with the 256GB SSD upgrade.
The default 5400 drive is dog slow, and the fusion drive upgrade is not much better, since that Mac can only upgrade to 1TB which has a tiny SSD.
I don't see that on Best Buy's website. The reason he wants to buy from Best Buy is that he has a give card. Would he just be able to purchase AppleCare for iMac from Best Buy? I don't see that as an option either on their site.
After the update today--now what should he buy? I was thinking the 4K 21 Inch with Fusion drive?