Here are some tips that might assist you in finding the right company
Look for a company that is established and has a proven track record. You want experience and don't want to fund their learning curve.
Look at their portfolio and make sure there is variation in what they do and that they have a good mix of clients. When it comes to web there is no one shoe fits all kind of solutions. The solutions they build should suit and deliver to the respective industry audience.
Make sure they offer all web marketing services. This way you can be sure they will not pass the buck and blame someone else if the solution is not performing. It will also ensure they know how one aspect of web marketing influences others within the web marketing space.
Make sure they have definite process in place for web design, website development, project management, testing, delivery and ongoing support.
Ask questions... if it's not clear to you what you are getting, get clarification. If still not clear... then follow your instincts.
Ask about ongoing cost. You don't want to lock yourself into a solution that is going to cost an arm and a leg to maintain.
Make sure you own the solution and can be moved elsewhere should you require it. Many companies provide a solution that is not transportable.
Make sure you will be able to update and maintain the solution yourself (if this is something that is important to you).
Ask about the development technology used. Will other companies be able work with your solution should you need to change providers.