Let me try another approach. We have three needs: a top corner, a rotational DE and a sub DE.
A) We need a #1 corner. If we plan to draft a corner, then we still need a #1 corner. Dennard is not that, and a rookie is not that in Belichick's system. I think that there would be a MAJOR reduction in 2014 quality if we replaced Talib with a rookie.
B) I am NOT talking about a marginal replacement for Bequette who would be in inactive in most games. I am talking about someone to put in a 3-man rotation with Ninkovich and Jones.
C) I am not talking about a marginal upgrade to Carter/Buchanan. I am talking about about someone who will actually help the defense.
BOTTOM LINE
We had three positions filled by players who produced little. I want to have 2 players who are actually likely produce plus a third developmental player (who could be an incumbent).
In additional, the rotational player need to be capable of starting and getting 90% of the defensive reps if Ninkovich or Carter is injured.
And yes, I understand that Belichick will figure out how to deal with injuries, even if we don't have adequate replacements. It still should be a GOAL to have these players.
Since need (A) in your list was filled by Talib in 2013, I find it hard to say that need was "filled by [a] player who produced little". Also, I'd say we have 4 needs - the ones you mentioned plus at least one stud in the middle who can draw double teams, stop the run, push the pocket and ideally provide some interior penetration. Call that one (D). It's not clear that that player is currently on the team, including Wilfork.
My take on those 4 needs:
(A) Re-sign Talib. Any other option - FA or the draft - results in a potentially significant drop off in the level of the secondary, IMHO.
(B) Draft a rotational player. Signing a quality FA (Michael Johnson, Greg Hardy) would likely be prohibitive. Trent Murphy would be my choice, but there are others. It's also possible that Dont'a Hightower could be moved up on the line, but that would create needs elsewhere.
(C) Buchanan could develop, and Jamie Collins (or even Hightower) could play that role to some extent. But I would love to draft another player who can play in space but also play the sub rusher role. Marcus Smith seems like a good fit, but this draft is deep in sub rusher/OLB possibilities (Dee Ford, Kyle Van Noy, Demarcus Lawrence).
(D) Cut Wilfork and sign a UFA like Linval Joseph, or draft someone high like Timmy Jernigan or Ra'Shede Hageman, or both.
The other significant needs I see are another LB who can play in space (could be filled by the same player who plays the sub rusher role), an upgrade at S opposite McCourty, and depth at outside CB.