Q: Can I use iCloud Mail for business?

I don't see why not; they do have parameters/maximum sizes (like most email providers do):

Mailbox size and message sending limits in iCloud - Apple Support

I have not heard of a limitation (except for size) - iCloud is an just a web based email provider.

You could use it, how ever if privacy is one of your main concerns there are services like Hush mail that offer privacy mail to companies and business

It's up to you. If you are not working for a military/security/banking company (in which any case they would give you the email), iCloud email should suffice.

Every email is "read" in some fashion while traveling from your machine to another. That's how items can be marked as junk or spam before you even open them.

Your ISP is more likely to snoop than Apple.

Certainly you can use your iCloud email for your business: but the service is not positioned as a business service and has no guaranteed uptime and very limited support. Furthermore you are showing your clients that you are using a free domestic service, which I wouldn't have thought was a recommendation. A properly hosted business service, with a guaranteed uptime and good telephone support, and using your own domain for the address (domains aren't at all expensive) would seem worth considering.

Do you have a website for you business? If you do you can have the provider setup email service with your domain name as the email provider.  In other words if you business website is AbrahamGrit.com the email address can be XXXXX@AbrahamGrit.com.

Spend some money and some time learning.