Appropriate to start business email or letter with just "Dear,"?

Dear is a perfectly appropriate letter greeting in all circumstances. However you do need a noun to follow. "Dear, " on its own doesn't work.

The standard opening if you don't know enough about the reader is "Dear Sir/Madam,"

Note that when used to open a letter, dear is an adjective:

Dear Mr Smith,

Dear Susan,

Dear Sir/Madam,

Dear is also used as a noun, typically in conversational speech:

Yes, dear.

Calm down, dear.

This usage is usually reserved for loved ones -- but this is a completely different usage from the letter opening greeting.

Dear on its own is not a valid opening for a letter, whether personal or business. I have seen this usage quite a lot, though, in messages posted to online groups by people from India. I suspect this is commonly taught there, but no native English speaker would use it.

If you address a letter simply as "Dear,", you are actually using a vocative to a person you are calling dear. This would be the same as starting a letter, "My dearest love," such as the famous Sullivan Ballou letter.

I can think of no case in which a formal letter, sent to an institution, would would ever make logical sense.

That said, if you started a personal letter with "Dear," you are calling out to your dearly beloved.

Also note, if you are writing a formal letter, the salutation should be followed by a colon, not a comma. Thus,

Dear Snookums,

but

Dear Sirs:

(Or for that matter To the John Deere Company: or To Whom it may concern: )

Of course, if you follow the link, you'll see that Sullivan Ballou himself actually punctuated his very personal letter with a colon. This is no longer appropriate in Standard English.

No, it isn't (unless you are addressing your sweetheart). The word "dear" demands a complement.

If you don't know who you're writing to, the traditional salutation is

Dear Sir,

These days, to avoid sexism, most writers prefer

Dear Sir / Madam,

Alternatively, you can address your letter to the group of people you are addressing. E.g.

Dear Purchasing Department,

Dear Stack Exchange Moderators,

Finally, don't forget that when you close a letter, if you don't know the name of the person you are addressing you should sign it:

Yours faithfully,