I'm building a forum, hopeful one day I will have as many posts and members as this forum. The problem is that I'm not a web designer or developer. Do I need a webdeveloper? I would think starting with a template and edit from there would be the easiest way to go?

I have no problem hiring one to get started, but as I add topics I have a feeling that I will need to make frequent updates. What is the best way to do this?

For example, If it were a forum about cars and I started with topics on Ford and Chevy; would I be able to add weekly updates myself for Toyota, Ferrari, etc? Or will I need to pay someone everytime they make updates.

Well, webhostingtalk's website goes directly to the forums, why don't you do the same? That way, there wouldn't really be any design you'd have to do. Also, forum software is pretty much easy to add and change categories, I don't see why you would need a web developer. Find some forum software and read through the documentation, changing categories or adding them is no sweat.

With a forum you should be able to post anytime you want. If unclear, why not try out some of those free forum hosting out there? This will prepare you for your own forum later on.

There are a lot of forum softwares like PHPBB, Vbulletin etc which are just ready to use. And many of them are free too. As US-Bobby said, if your site directly loads the forum you won't be needing any designing stuffs.

I would think starting with a template and edit from there would be the easiest way to go?Do you really need to go into the editing part? It looks to me like you're over complicating yourself in these early stages. Start small and simple, see if you have the audience you expect etc. People stick to forums due to activity and otherwise quality new content, much less so due to how the forum looks.

The idea of playing with some of the free forums for a few days seems very good to me. I recommend you take a look at simplemachines.org.

There are a lot of forum softwares like PHPBB, Vbulletin etc which are just ready to use. And many of them are free too. As US-Bobby said, if your site directly loads the forum you won't be needing any designing stuffs.
Maybe I can add SMF for consideration www.simplemachines.org/
The features is complex and many free NICE templates there