Hi Kim, here are a few thoughts about your questions...
Get them to show examples of their work. Have them explain what they did for other customers at some level of detail to make sure they can articulate what they are doing to your satisfaction. If you've got a developer who you do trust to do the work and who simply doesn't have the time, maybe have them on a call to ask more technical questions. Finally, be sure to review example sites and ask for references.
I would also say that there's a great difference between website designers and community designers. It's a very different orientation toward interactivity, usability and content, in my experience.
Depending on the platform you choose, that will inform whether you are best using an in-house resource, a contractor, the platform vendor's professional services, or a vender implementation partner. If you go with an open source platform you'll most likely be looking at an independent or someone in-house. Most of the large enterprise platform vendors have their own professional services group or implementation partners who can help you.
There really are quite a few options, so I sympathize with how overwhelmed you are feeling. From open source platforms through enterprise grade, there is a very wide range of pricing options for you to consider. Start with your business objectives, the target audience you want to reach, and the activities that audience is going to want or need to be able to do there. If you start out just chatting with vendors, you are far less likely to end up in a good place.
If you'd like to talk live about this in order to dig in further, I'm available for a conference call or Skype. Send me a private message and we can set something up.