CvZ is one of those games that penalizes slow languages, unfortunately. There are a few (not many, but some) Python3 entries in the top-100 of the game, but none in the top-50. So it's definitely possible to do something decent in Python but don't plan on doing something awesome. That said, there are close to 3,000 other players in the leaderboard, so top-100 is nothing to sniff at.
I don't know for sure how those Python bots work but I'd be amazed if they aren't doing a simulation and some form of search.
Not sure what a reasonable number of nodes would be for a Python bot. For reference though, my C# bot runs around 250k round simulations each turn (and it's not even all that good). Don't forget though, that a good algo in a slow language will beat a bad algo in a fast language most of the time. i.e. what you do is usually more important than how often you do it.