Well, you might still pick one of the open source solutions and keep upgrading your Q&A site as the software matures.
If you need more info about askbot - that's the one I am developing - just drop by at the site and ask or email firstname.lastname@example.org.
With askbot upgrades are normally painless and the product is stable. There are more test cases than in any other similar product I know of, no known critical bugs and the development is steady.
Of course there are other open source solutions available and you know where to find them...
Also I want to add that I am gathering support team right now and I might be able to match your needs with the developers who can execute them and at the same time contribute to the main project.
edit: I want to defend open source solutions here. They give an advantage to the client - not to depend on a single entity for the availability of a service or a product. IT business is quite volatile - StackExchange 1.0 is one example. Secondly - I just do not believe that proprietary products of this class can be sustained at a higher level than the open source in the long run - because forums are relatively simple.