Just re-read your request and realized you want to add a check on that field.
I'd still recommend what I wrote above, but change the text below the field to 'Word count (minimum 150): ' or similar.
Also, update the JS to this:
(function() {
function wordCount(essay) {
return essay.split(' ')
.filter(function(n) { return n != ''})
.length;
}
$(document).ready(function() {
$('.Submit').attr('disabled', 'disabled');
$('.essay textarea').on('input', function() {
var count = wordCount($('.essay textarea').val());
$('.essay label.cf-helptext.un-help').text('Word count (minimum 150): ' + count);
if (count >= 150) {
$('.Submit').removeAttr('disabled');
}
else {
$('.Submit').attr('disabled', 'disabled');
}
});
});
})();
I think that will be a little closer to what you're looking for.