Hey HotPasta - sig's and avatars are set in the "User CP" section of the site. Some of the features (custom avatar?) are only available with donation or if you are a "site supporter". I'm not sure which features are available at what level, etc, maybe someone in the know can add additional details...
I think the purpose is not to ask anyone to pay just so they can have a signature or custom avatar.
If you like the site and find it helpful and useful, not to mention saving time, money and effort from the members' experience and expertise (e.g., DansSLK), and the camaraderie and you decide to voluntarily support the site by getting a premium membership, then as appreciation for your support, you then have some "perks", like custom avatar, signature, signature pix, etc.
This is such a great site and I have learned so much about my car. More than I have ever imagined! I myself and debating on which type of donation I would like!!! I definitely wouldn't mind supporting this site!