It's the kind of thing you would write in an official letter. However 'i look forward' is more formal; Onward sounds odd and is uncommon at the start of a sentence.
From beethoven onward, composers notated dynamics in a more direct and detailed manner. As at would mean at a precise time of event. If you mean both in the sense of anticipating something, both are equally valid.
Here's the definition of onward in the oxford english dictionary, adverb, entry 4b. 1 as of would mean at a certain time onward. Backwards) to be used as an interjection, so there's nothing wrong with using it as a single. Apparently, there is a slight difference, but only in british english:
What is another word or phrase (other than since) for from. As from would mean at a certain time onward just like as of, but i still don't quite get it.