In our operation we have an automated flightplanning system(it checks route, wx, runways, notams, etc) and it will automatically select the least fuel alternate. The system does not look at commercial implications. Only if the weather becomes at or below minimums we have to call dispatch and look at other factors taking into account the commercial operation. On the distance, there is no minimum distance but there is a max distance. For "two engine operation" you always have to be within one hour(no wind) single engine flying time of an adequate alternate. Unless of course you are ETOPS approved.
Greetings, Mark