A bit of PHP knowledge is always useful before launching into the world of full-blown ORM/CMS's. However:

These are the features I need to begin with:
-Only admins can create users

You can configure which groups can/can't access the security panel. So yes, you can configure it so that some user groups can manage users and others can't.

-Public and user-only content

Yes, you can configure any page or part of your website that only certain groups can view.

-User-group permissions

Yes

-Custom content types with pages to list specific types (Like Drupal's CCK and Views)

Not sure exactly what you mean, but with some limited PHP and some templating skills you can create as many page types as you want.

-File attachments

Yes. Note that you'll need a small amount of PHP to set this up.

-Contact form

Yes, with the userforms module, or you can write this with PHP fairly easily.

-Newsletter

Yes, with the newsletter module.

-Forum

Yes, with the forum module.

Some questions I have regarding SS:
-Would the subsites be suitable for unrelated sites? I guess I would want to use it more like a multi-site, single codebase. I read in another thread that multisites can be done, but that might not be wise for a newbie like me.

Not really suitable for newbies .

-How complex is it to install new updates? Drupal drives me nuts having to disable all my modules, themes then re-enable them after

Adding modules and themes are as easy as uploading them to your server and running a 'build' process (takes a few seconds).

Installing updates depends on the update. I use Subversion to update from the SilverStripe svn, and it is usually very easy, but there is no 'simple' process as yet. One goal of the next major release is an update manager that will streamline this process.

That said, if you are not building any complex modules or writing a lot of PHP, the upgrade process should be simply upload and run the build process. No guarantees there though

-How easy would it be for a complete PHP newbie to learn to add to SS? How is the documentation compared to say Drupal; which I found to have a void between beginner and developer? I notice you have a book coming out.

SilverStripe probably wouldn't be the best way to learn PHP, the documentation usually assumes some basic knowledge. That said, you should be able to do a lot of what you want without any PHP, and the documentation covers your needs fairly well. Personally, I've found SilverStripe much easier to learn and use that Drupal.

-One future feature I would like to add to my site is having a single piece of content under many categories with multiple sub-categories, I suppose that might be quite complex, but would it be within the scope of someone like me?

I think so. The best place to start is the tutorials - they're quick and painless and will give you a good idea if SS is right for you:

http://doc.silverstripe.org/doku.php?id=tutorials