1. No, all emails to your account will come, unless you filter them, and I don't believe filtering based on attachment is available.
2. No, BUT, your BB will only download the "headers" of the emails it receives. You can easily delete those with large attachments (or never open the attachments) and not use that bandwidth.
I would suggest getting the unlimited BB Data Plan if you are overly worried about additional data charges. Likewise, setup a better spam software on your account and keep spam from coming in.