With Sexson instead of Dunn at 1B, then you might have the best infield in the league. Dunn's OK for a utility spot if you can take the hit in average, but there are quite a few 1B I'd want ahead of him if I was going to make the claim for the best infield in the league. Blalock isn't much of a drop-off from Lowell though, so I think you'd get the better of that deal.
I agree with the assessment that your biggest need is pitching. Wagner won't win you saves alone against teams with multiple closers, so you need someone better than Koch (is there anyone who isn't?) to support him. And your starters are OK for No. 3-6 types, but you need a legitimate ace or two to anchor that staff. You'll have to give up an offensive starter for those kind of pitchers though, as no one on your bench will get you a top starter. Even though I like Sexson over Lowell, you might be better off cashing in that chip for pitching instead of more hitting.
"I'm telling ya, he jumped us. Gloves off, stick down, no warning. He challenged the Chiefs. Called us names. But Dave was there."
"Dave's a killer."
"Yeah, Dave's a killer."
"Dave's a mess."