Prospective PhD advisor encourages me to apply and positively mentions relevant experiences. What are my success chances?
The professor's asking to be kept informed, which is certainly a positive sign because he's interested enough to want to know what you're doing. At the very least he's not outright rejecting you.
On the other hand, there is no guarantee he will take you on until you actually receive the offer letter.
I don't think there's much more to say other than thank him, reaffirm that you will submit an application (if you are planning to do so) and say you will keep him informed when you do.