there is not minimum prestige. c- can pull down. however, within 70 miles just gives you a BONUS, and there are NOT many recruits within 70 (or 50) miles. there is nothing special about the 50 mile mark, although some coaches say teh closer within 70 miles, the higher chance someone will talk to you, but ive never checked that myself.
anyway, the trick is to send a bunch of guys calls. i used to send a couple HUNDRED per season, but now that FSS exists, i probably only send like, 50. try sending out calls to anyone who looks decent in d2, who is like, bottom half by overall rating or so, and you'll get some yesses back.