All depends the amount of traffic that you may expect in your website, if you think your website is going to be visit by 20 people a day, then the Silver option will be ok. But if you are expecting more than that then you should choose the Gold option. You always can change the plan according to your needs.

Disk space: it depends of the size of your website, most websites are under 5gb of space, Bandwidth per month: if your site has a lot traffic or big file that people will be downloading, then that will imply that your traffic will increase a lot. If you have a basic website it should not take that much.

E-email account: if you have a lot of employees you may need one account for each one of them as well, and with a lot space for each account, it depends of how long do you want to keep the emails in your inbox.