should 下午 be able to come either before or after 我们？
I'm not a native speaker, but from what I understand, I'd say yes. Words for times (like 早上，下午，晚上，etc.) can come before or immediately after the subject.
"我们下午去商店买礼物" is accepted.
Shouldn't it be 这个下午我们要去商店买礼物。?