A lot of WordPress sites seem to be nice and snappy after a re-design, but something seems to happen to them over time. E.g. Balloon-Juice was amazingly fast after it's recent re-design, but over time has it developed things like reader responses showing up 3x per submission, creaking slowness under higher-than-normal loads, front-pagers being unable to embed videos for unknown reasons, etc. (It seems like stopping and restarting the commenting database part of the site helped some for some of these issues, but that's obviously not a permanent fix.)

IOW, I wouldn't count on a one-time clean-up of the code. I'd think you'd want to find someone who can de-lint the site and also be available for tweaks and fixes a few weeks/months down the road when the inevitable storm of FYWPs hit. But maybe you can handle that after the initial fix (of course finding time for that is a problem...).

My $0.02. Good luck.

Cheers,
Scott.