In my opinion, I think it is likely that Buddhism was a fairly well established religion before Ashoka leveraged it for his own uses. Perhaps it was somewhat localized, but established nonetheless.

Why do I think this?

• Ashoka's main motivation for propagating Buddhism was almost certainly a type of marketing. After brutal campaigns of conquest, he wanted to show a "newer and softer" side of himself and Buddhism fit the bill nicely. If this marketing was indeed his motivation, picking an obscure religion wouldnt be very productive towards this aim. Picking something that is at least somewhat known with an existing and well understood philosophy and ethical basis would be the best approach. Just the word "Dharma" conveys alot of information to those he was communicating with. Lets say instead of Buddhism he picked a lizard worhshiping cult. If he made a bunch a pillars bragging about his obscure lizard worshiping ways, people would just be confused and no implicit narrative of ethical stance would be conveyed by this choice. In our own time if a politician proclaims that he or she is a Christian, there is alot of implied information in this proclamation regarding this person's values and belief system. But this information is only implicit if someone knows about Christianity to start with.
• The Ashoka Edicts reference Buddhism in a way that seems to imply existing context. There is no introduction of Buddhism but rather a referencing of specific aspects with the assumption that people will know what it being talked about. If Buddhism was not a well know religion, there undoubtedly would have to be text on the Pillars explaining exactly what Buddhism actually was.
• If you look at the introduction of Buddhism to China you will see the the religion was first propagated informally by merchants and then only later by rulers. It seems plausible that Buddhism would follow this pattern earlier in its history as well. (See "The Mauryan Horizon: An Archaeological Analysis of Early Buddhism and the Mauryan Empire at Lumbini, Nepal" for a scholarly treatment of this idea: http://etheses.dur.ac.uk/11038/)
• Ashoka states explicitly what areas he has attempted to spread buddhism to. If he had spread it to the core places within his vast imperial control, he undoubtedly would have included those places in his list. If those locations are not in his list, its fair to assume Buddhism already existed there to some degree.
• There seemed to already be a good amount of man power and evangelizing methodology that Ashoka was able to make use of in his attempts at propagating Buddhism.
• Lumbini (Buddha's Birthplace) shows archaeological evidence of already being a developed pilgrimage spot in pre-Mauryan times. (See "An Archaeological History of Indian Buddhism " by Lars Fogelin. Also: o https://news.nationalgeographic.com/news/2013/11/131125-buddha-birth-nepal-archaeology-science-lumbini-religion-history)

So in short, Ashoka certainly was instrumental in propagating Buddhism further than it probably would have otherwise, but only a somewhat established religion would have sufficed for his needs.