Answering a) and c), because I have no experience for b).
a) Yes, Discourse is very responsive. The apps are relatively new, and blocked for 100% new users. Users need to be active enough (I think it is read three topics or similiarly simple) to not be “trust level 0” to use the app. Before that, they can use a browser on a phone quite easily.
b) There are several SMF import topics, you should search and look at those.
c) Discourse can be self-hosted on Linux cloud instances in the $10/month range. (It could be self-hosted on a physical Linux box just as easily.) It needs to be an system that supports Docker, which rules out some VPS providers. There are also options to have someone else manage it for you, like the Discourse.org hosting and some other providers. I believe Windows is only supported as a development environment not a production one. I’ve never even tried dev on Windows.