In my opinion you should take a witch from start. (and 10 linemen)
Why?
The most important skill for a witch is of course block. when she got it she is very useful but before she does you have to really be careful when and where you decide to throw blocks and blitzes with her, so she is of limited use. Blitz skill can put you in deep probs sometimes, and will also make you loose some rerolls too if youre not careful, this is because the follow-up move is quite often a single die block and a 2/6 chance that she will go down and your turn ends.
So i suggest you get a witch from start, let her score twice and voila, you got a really useful player from game 2 and on. When u add your blitzers they will be useful from their first game.
Also, as Longshot implied, witches are the only thing that sets this team appart from similar teams, they are your ace up your sleeve.
Take a peek at my favourite player in my team:
http://fumbbl.com/FUMBBL.php?page=player&player_id=6776
she is a real thorn in the side of any opponent. Good luck !