As David mentioned the migration is probably the most critical piece to get right if you make the jump.
* Based on the priorities you outlined I believe this is a wise jump for you pending how you execute the new q&a forum.
* Using the right one will, there is a lot out there but the main thing you're going to want/need is a q&a that generates a unique post per q&a - if its a plugin that simply creates a thread of information or posts all on one page or just a few pages you'll be losing out on some of the main power WordPress can offer by migrating to it. You'll want to ensure that you also setup the hierarchy to support the SEO funnels you are trying to build. You'll most likely simply use categories inside WP to accomplish this.
* Without knowing the current format of your data or the site this is coming from it would be an uneducated opinion at best to provide a recommendation.
* This last question raises more questions than answers - are you planing to still maintain the ASP.net site or do a full migration, what will be the authoritative source of data, if your doing a full migration what authentication is necessary?