Just to chime in and hopefully help someone else on the thread; the best way is to use 'not equal to' instead, so you can catch all the higher level of wholesale customers too (not just the 1st level). Also notice singular 'customer_whole' (that may be different depending on your Dual Pricing version).

Code:
if ($_SESSION['cart']->count_contents() > 0 && $_SESSION['customer_whole'] != 0 && MIN_ORDER_AMOUNT > 0)