Buses run constantly from Athens to Nafplio throughout the day.
You should have been able to buy tickets on line but it's very easy to buy them at the bus station. Just get there ahead of time.
Driving in Greece is safe, the roads are good but paper maps, especially for the Peloponnese can be cumbersome and difficult to follow.
I used Google Maps for navigation in the Peloponnese and for the most part got me where I wanted to go.
You can rent a car once you get to Nafplio if you don't want to drive from Athens. As mentioned buses are clean, safe and comfortable.
Here's the best site for Nafplio as it covers everything you need to know and do.