I tried to find a replacement for myself at a firm where I was doing all of the online marketing for clients and it was really, really hard. First, don't take anyone without experience running profitable campaigns. In this business, mistakes can cost thousands. Can you imagine some idiot putting an extra 0 on a daily budget?
Good language skills are a must for keyword research, ad copy and page copy. You are also going to want to be providing detailed reporting to your clients. Not just stats, but detailed written analyses. Each month, I put together about 20 pages of stats and a 3-5 page written report. This may seem like overkill, but it's key for client retention. Clients knew exactly how the money was spent, what had changed, what the results had been, what would change over the upcoming month and what results we were expecting.
Also make sure you've got someone who can think way beyond the PPC aspect of the business. Where is the traffic coming from for each keyword or contextual ad? How does this prequalify the visitor? How will the visitor be converted into a lead? How is your client going to convert the lead into a customer? Can he put dollar amounts on all of this in his head and really find the strategy that will produce a good ROI?
Think of an example scenario and ask a series of "what would you do" type questions.
Now, get ready for a very long and painful process! I got hundreds of resumes and stayed on for about six extra months because I didn't want to leave that firm without a proper replacement.