1) How many nights would you recommend spending on Lake Hallstatt? What towns are the best for lodging and a base? Hallstatt? Obertraun? Bad Ischl?

2) Is it easy to get from one town to the other in the evening? looks like there is ferry service in the day?

We will be traveling to Salzburg for a night then munich for a wedding.

There is enough to see and do to keep you occupied for two full days in Hallstatt... depending on your interests. So, it depends on your arrival and departure times and your interests if one, two or three nights are ideal.

i think that 3 nights might be preferable to 2 if you have the time... especially if you want to hike or to visit the nearby towns. it's a beautiful place to just relax. for me, by the time i get there and unpack, a 2 night stay only leaves the day after arrival to really explore. i assume you are taking the train, then boat from salzburg... so that eats up a bit of time as well.

I think Halstatt's the best, but Bad Ischl isn't "bad" at all. I think Halstatt is at it's best in the evening and in the morning. I would also like to recommend nearby Gosausee, the Gasthof Gosausee is a brilliant hotel.
You can have a look at my page on Halstat: http://ashwinbahulkar.wordpress.com/...-and-achstein/

If you like walking and hiking, the area deserves 3 days.
We were in Hallstatt last week for 3 nights, 2.5 days of rain and very cold. We arrived mid-day from Vienna, visited the Catholic church and Bone Chapel and the excellent museum. We could not do any hiking- too much rain. Day 2 was for St Wolfgang, took the boat to St Gilgen and back; skipped the train up the mountain because of clouds (nothing to see according to the ticket staff). Day 3 was for Dachstein, but the Ice Cave and the cable car to Krippenstein were closed, dangerous conditions, but the Mammoth Cave was open. We were not interested in the Mammoth Cave and so took the bus back to Hallstatt. With nothing else to do, we went on the Salt Mine tour, and much to our surprise, it was quite good.