Hosting from home and having "no worries" are mutually exclusive. Arguably, running a website and having "no worries" are mutually exclusive... ;)
The security measures you need to take are securely configuring, and keeping up to date with security patches on:
1. Your router/firewall
2. You computer OS
3. Your web server software
4. Your db server software
5. Your forum software
Using a proper web host takes care of 1-4. This is why most people use professional hosting.
I suppose this really comes down to your skills and experience. If you're unfamiliar with securing and configuring a web server, are you prepared to put in the time to learn? Remember this is the time you would otherwise be using to build and run your forum.
Shared hosting is cheap. Your monthly electricity charges for keeping the server on 24/7 will probably cost more than a basic hosting package.
Don't forget the other disadvantages: what happens if a hard disk fails? Or your internet connection goes down for a day? Or your house gets broken into and your computer is stolen? Will you scrupulously avoid uploading/downloading large files when people are using your forum? etc. You also need to check that your ISP allows you to host websites on your connection - some block port 80, some disallow in TnC's.