A word of warning here Simon, whilst many designers can create great looking emails, all too often the campaign is won and lost on deliverability. This is governed by many factors but the designer will have an impact on content and maybe HTML coding.

It's probably worth getting a seasoned pro involved at least for a while, or you can really screw up your lists and have a tough job getting back into the inbox.

I've got a couple of names who won't break the bank if you're interested.

If you find someone who's got the brain for both creative visual design and for the analytics to keep improving the campaigns, I'd say you're a very lucky man.