This is an important job for many companies and Alice has written a great blog on how to become one, make sure see it at:
Becoming a technical evangelist
That is how Alice Pang became a technical evangelist. My road was completely different, as yours will be.
Many companies need people to explain how their technology works, and not have a sales metric. Sales people are the respected soldiers of the corporate world, focused, with a good work ethic, but once the sales is made, the relationship changes. With Evangelism, you don’t aren’t attached to commission, or a salary and commission.
What I like about evangelist is that I can build relationships and community.
You would have a different take on the situation!