Is this your company's first website? Do you or anyone in the company has any prior experience on web or wordpress? My suggestion is for you to find a hosting company on your local area or find a company where you can talk to them physically.
As for your other concerns:
How many servers?
- It depends on how content, concurrent users your site will have
How will we handle updates?
- Unless, someone on your company knows how to, or is willing to learn then your company can update it.
Who will support it?
- Again same as the above. But like this forums, you can find many other websites where you can ask questions and such.
What if the plugins for active directory stop being developed?
- Now this is the hardest part, as you know plugins listed on the wordpress.org directory doesn't have any guarantee of support or update. It is available as it is, and it solely depends on the plugin author to provide support or update. You can either hire someone to update the plugin if the original author abandoned it or wait for someone to continue it.
Hope this helps =)