I might agree with your philosophy of making the playbook fit the team's personnel if said team has elite personnel. We obviously do not have such a team. The only players currently on the defensive front seven worth building around (should still be here three years from now) are Kyle Williams, Paul Posluszny, Aaron Schobel, and maybe Aaron Maybin. To go through your list of players that the Bills have made financial or developmental commitment:
DE Chris Kelsay - * - Does not fit in the 3-4 defense. I believe his undeserved large cap hit ends after next season.
DE Aaron Maybin - *^ - Fits better as a 3-4 OLB than 4-3 DE.
DE Chris Ellis - ^ - Does not fit in a Buffalo Bills uniform. He has rarely been able to crack the lineup. It is nearly safe to call this draft pick a bust.
DT Marcus Stroud - * - He is 31 and his play has been on a sharp decline, yet was recently given a four year extension. He is about the same size as Richard Seymour. He should fit as a DE in a 3-4.
DT Kyle Williams - ^ - He is more of a penetrator than a gap/blocker eater, but he could probably transition to a DE in a 3-4.
DT Spencer Johnson - * - DE in a 3-4
DT John McCargo - ^ - He was given a second chance with a new DL coach, but didn't take advantage. I don't see him being kept much longer.
LB Kawika Mitchell - * - ILB in a 3-4
LB Paul Posluszny - ^ - ILB in a 3-4
LB Nic Harris - ^ - He is probably too small to play OLB in a 3-4. I haven't seen enough of him yet to consider him a valuable building block to a defense or not.
I left Schobel for the end because his situation seems more complicated. I believe he could transition to an OLB in a 3-4 if he was motivated to do so. Despite another productive season, he revealed that he is considering retirement, citing a decline in athletic ability. Being with a team with one winning season in the past ten years and no playoff appearances has a lot to do with it, too, I'm sure. Forcing him into a position that requires even more athleticism while learning an entirely new scheme, may push him more towards retirement or a trade request. Schobel gets a lot of criticism for being "over the hill" and for his large contract, but he has a been our most productive defensive player this decade, continues to be our best defensive linemen, and should have a few more years in him.
It's true that a transition to the 3-4 would require massive roster turnover, but with a new regime taking over when their predecessor has failed so miserably, a house cleaning is likely to happen no matter what. If such a change were made, the main deficiency the current roster has would be the lack of a true NT. There is no one on the current roster that would fit that role. Nix would need to acquire two huge space-eaters to rotate at NT, at least one of which coming from the draft. These types of players are rarely found in free agency (not enough Tom Donahoe's in the league) unless they are 35 years old. NT would move up in draft priority shortly behind OT and QB.
To answer OP's other question, you can see each players contract details on their rotoworld profile.