Here's my answers to your questions:
1. You can always set in your (Site Admin Menu) for all your site contents (blocks, modules, etc...) to be visible to all & not just for registered users. As for your Forum, you can set your Forum settings (under Modules Admin Menu) by setting permissions on it either making it Public, Registered, Registered (Hiden), Private, Private (Hiden), Moderators, Moderators (Hiden). You can also create (User Groups) to have particular access to a particular block.
2. You can also set permissions for your Download module similar to the permissions mentioned above depending upon what kind of Download module you're using (ex. NSN Downloads). Uploading is allowed depending on how you set permissions on your Download module. It's for you to review & accept the file as site admin. There's a hack that you can find in the net that makes your Downloads visible to all yet only a particular someone with the right permission will be able download.
3. You need to have a subscription module if you want limited access to your Downloads for Subscribers. Most popular subscription module is Western Studios' Subscription module version 2.0. To get this, you have to subscribe at Western Studios - Free and custom PHP Nuke themes, graphics designs, tutorials or get the free version 1.0. There's also a hack out in the net for you to set limits on how many dowloads a registered user can download in a day or week. You have this option in WS Subscription module 2.0.