If you have few or no IT staff and plan to do some of the administration yourself we recommend Windows 2008 SBS Server at this time.  The product is relatively stable and has matured.  If you have some IT staff they must be familiar with it.  Otherwise budget for and negotiate for external support.  Once it has been set up it should rarely go wrong.  An engineer can cost around £60 per hour, but should never be required for more than 3 hours.  Get prices in advance. Most problems are created by people messing with the system without the proper training who do not document what they are doing.