Shared hosting might not be the best long-term solution for forums.
Shared hosting doesn't allocate individual resources for each users. That means you will be sharing resources for all your server neighbors. If one of the neighbors use a lot of resources, then your forum would not have sufficient resources to perform optimally. This may affect your forum website loading speed and the overall perception from your target audience. If your target audience is required to refresh a lot of times or wait for a long time to register as a member, to view a post or to post a reply, that gives a bad impression for your target audience. Eventually they move from your forum to another forum with a similar niche.
I recommend looking into VPS. You can start with a basic plan and scale up from there when necessary. Both Windows and Linux allows you to scale up, when the time comes.
If you are targeting a world wide audience, then Content Delivery Network (CDN) might be a good solution. CDN ensures fast loading speed, no matter from where your audience comes from. Instead of renting multiple servers from multiple locations of the world, CDN is a better solution.
If you are targeting a localized audience, then make sure that the hosting company that you chooses host their servers within the proximity of you and your target audience. For example, if I am from Malaysia, and my main target audience are Malaysians, then it is best that I subscribe a hosting plan from a company that hosts servers in data centers in Kuala Lumpur (Malaysia).