Thinking to replace my existing q&a system with wordpress plugin for Q&A considering SEO as first thing in mind, pls suggest is my decision right?

You are right. Any updates on site will require you to think about SEO so there is no lost traffic

here is a QA wordpress you can use http://www.designwall.com/wordpress/plugins/dw-question-answer/

Also this is good seo plugin as well https://yoast.com/wordpress/plugins/seo/

It's very important that, not only the system you are willing to use is important, but also the transfer to this system. How much URLs we are talking about? 1000, 10000 - 1.000.000 ?

Wordpress might get heavy and might need a special setup for high volume content sites, or high traffic sites. Changing the system is a huge risk that should be evaluated before doing the change. Remember to keep track of each single URL and redirect accordingly to the new URLs with a 301 in case the URLs really HAVE to change.

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?