Is there a setting to ensure all emails received with attachment is fully downloaded ie with the attachment

Mail
1

Downloading email attachment completely ,as email arrives

2

Yes, you will find this in Menu > Tools > Accounts and then in the IMAP tab for your email account.

POP3 accounts download the content automatically, so there is nothing to change if you are using POP3.

Either way, this does not affect linked content, and that will still need to be downloaded every time you open the message.