What Marv is doing is the correct way to build a team - start with the lines and then work your way down to the skill positions - with the walker trade we now seem to be set on bot sides of the line thus I don't see marv taking another lineman (O or D) unless okoye is there at 12 and willis isn't.
With the no. 12 Pick I definitely see Marv taking Willis - the guy is a play maker, he's fast and strong, and importantly for the defense he can shed blocks and attack a runner at the line of scrimmage - he's also an incredibly high character guy which I'm sure endears him to Marv - he could be a cog on the bills defense for years to come. I might even see marv throw in a 4th round pick to move up two spots to grab this guy. LB is our weakest position right now. Crowell is a great LB. Ellison is pretty good too, but he is sort of undersized, which is not such a problem at WLB IF we have a good MLB. Coy Wire was kept because he is an outstanding special teams player, and although he will likely see some time at LB - especially in obvious pass situations - he is definitely not the answer we're looking for.
If Willis is not there at 12 and Okoye is, I'm pretty sure Marv would take him - this guy has the potential to be the next Warren Sapp and he's only 19 - If you remember correctly the cornerstone for the Tampa Defense - on which we are basing our playing now - was Warren Sapp. If this happens I could envision a rotation with Walker McCargo Tripplett Okoye and Kyle Williams with Anderson being released. If you're lineman are great and all over the QB linebackers are left free in the backfield and corners are not forced to cover their men for long periods - thus average corners could do an outstanding job.
As for RB I don't see us drafting one until late the first day either - with the revamped Bills O-Line, a RBBC approach should be serviceable if only for one year. The A-Train would be a serviceable starter - he hits holes well and fast, he is an excellent blocker and an above average pass receiver - and hopefully the holes opened up by the new line would make my grandmother seem like the next Jim Brown - although he is not going to break a long one he should be good for around 4 ypc which is pretty decent by any standards. Remember next year Michael Turner becomes a free agent, and he is the one RB I've seen that truly reminds me of Thurman Thomas.
If neither Willis nor Okoye is there at 12, I see Marv trying to trade down and grab another first day pick and select another LB later in the first round, someone like Poz or FSU's Timmons. In this scenario I wouldn't be surprised if he chose a corner - someone like Arkansas' Chris Houston.
As for RB I see us grabbing a fast, versatile runner like Kenny Irons with our first third rounder. We also need a serviceable no. 2 WR and this draft is very deep at that position.