Hey,
I've not done a boardmod/tested this but this would basically work:
Open poll.php and search for:
if ($username == 'Guest')
fatal_error($txt['yse28']);
$request = mysql_query("SELECT votedMemberIDs, votingLocked FROM {$db_prefix}polls WHERE (ID_POLL='$poll' AND FIND_IN_SET('$ID_MEMBER', votedMemberIDs)=0) LIMIT 1;") or database_error(__FILE__, __LINE__);
(it's just after the vote() function!)
Afterwards add:
$request = mysql_query("SELECT posts FROM {$db_prefix}members WHERE memberName='$username' LIMIT 1;") or database_error(__FILE__, __LINE__);
$tmp = mysql_fetch_row($request);
if($tmp[0]<MINPOST && $settings[7] != 'Administrator' && $settings[7] != 'Global Moderator')
fatal_error('You have not reached the required amount of posts to vote in this poll');
Where MINPOST is the amount of posts they must reach (ie replace it with whatever amount you want). I know I haven't made this into a mod etc but there should be enough here to allow you to try it yourself. I BELIEVE I have got the mysql request right - if it doesn't work it may be realName= instead of memberName=

I know this isn't exactly a full mod and you would prob want to add the option to adjust it form the admin menu etc but it will hopefully get you on your way
