How challenging is it to get to Sounion from Athens? Trying to decide whether to rent a car and go on our own. Or to hire a driver for 150 euros for 4 - 5 hours. Would also go straight from the airport from there.
Here and here are companies offering half-day tours from Athens to Sounion for a lot less than 150 euros -- but sharing the tour with others, of course. I can't vouch for them, haven't done the trip. But it wouldn't be challenging at all with a tour, and I doubt if driving your own rental car would be difficult either (outside of Athens itself) if you're planning to drive at all in Greece.
See Matt Barrett's Guide for more info on Sounion and links to other possible tours. Trip Advisor should also have reviews and ideas.
Whether a tour would pick you up or drop you off at the airport is another question, maybe not. Your reference "from the airport from there" is a little unclear.
Check Withlocals. It worked out great for us to get to get to Sounion with a local driver. I believe we paid $100 euro for four people from 2:00 PM - 9:00 PM.
Very easy. The hardest part is navigating traffic in Athens :-). If you are renting from the airport it's super easy; otherwise just make sure you have clear instructions as to how to get on the highway.
There is a cheap (6 euros when I went) bus from Athens to Sounion that I highly recommend. It takes 2 hours each way, but the drive is quite picturesque. It's not the fastest method, but it is the most cost effective. The bus leaves from Pedion Areos Park bus terminal near the National Archaeological Museum.