I wouldn't want Terry at Arsenal even if he was the best defender in the world ever.
Ade seems to have second season syndrome so I'd give him at least one more to see if he recovers the form of last season. RvP stayed fit a lot longer this season but seems to have lost his form a bit to compensate. I like Bendtner and he's constantly improving. Wouldn't want to see any of them leave TBH.
I'd say we need an out and out goal scorer but I still think that's Eduardo and he's been injured most of the season. Still, he managed 3 goals or so in the handful of games he did play which says a lot.
For me, our best bet is to go with one big striker (Ade/Bendtner) who can bully defenders, win balls in the air and score from crosses/corners and partner him with a smaller, more technical striker (RvP/Eduardo) who can go past people and score both good and scrappy goals.
We also have Vela (who's awesome), Theo might well move there when he learns how to take his time and finish consistently and, as you say, Simpson is waiting in the wings.
I'd say we need a left winger to free up Arshavin but I guess Nasri plays well there.
Not sure about central midfield. Cesc has to play obviously and we have Song, Denilson and Diaby all of whom I like (even Diaby, when he's not fucking around). The thing that needs to change, regardless of whether we buy anyone or not, is whoever plays the holding role needs to actually hold and not join in every attack leaving the defense vulnerable.
Personally I think all three are capable of holding their own physically but it would help having someone who can not only do that but is experienced enough to bully the opposition midfield and hold the team together when things go wrong.
I think we're OK at full back but the middle of defense is a worry. Silvestre seems past it unless he makes a remarkable return to form. Kolo and Bill are both good defenders but they both play better with a partner who's bigger, stronger, wins everything in the air and is generally very hard to knock off the ball thus leaving them free to use their pace to mop things up. Djourou is very good and fills this role to a degree but again he lacks experience.
If we're buying, I'd say a big lump of a central defender and a big lump of a center half who's only worries are stopping the opposition playing and giving the ball to our play makers of whom we seem to have about 137.