the tickets sold onboard , round trip, they didn’t Accept any kind of credit card ( just cask) , 5 min will be enough before the ferry move to train station, and, there are time before the last train arrived( I remembered that we waiting aroun 10min before the last train arrived)

and would it be worth an overnight stay or would just a day trip from Vienna or Salzburg be sufficient?

Hi, having just recently returned from staying near hallstatt, I personally would recommend staying close to hallstat rather than salzburg. Although we hired a car it is possible to see an awful lot in the area by either train or bus. We stayed at Au ( 2 mins walk fro Steeg gosau railway station) in a lovely apartment with a balcony great views. Halstatt was a 2 hour walk away around the East shore of the lake which I highly recommend for all the family. You can get there by train and bus the bus is free with the salzkammergut card ( this also gets you into lots of attractions free or gives a discount). If you get the train you also need to get the ferry across to halstatt cost 5 euros each way, great view of the town. We also did a couple of day trips without the car, one was to salzburg, the easiest way to get ther is by bus, from hallstatt to bad ischl where you change onto the 150 bus to salburg cost 11euro pp each way. Salzburg is a great city, highly recommend the fortress and the gardens, we only had 5 hours there which I felt we could done more with more time. We did also do a train journey to gmunden lovely place on a large lake. Unfortunately I cannot comment on railcards, but based on the prices we paid again gmunden was 9 euros pp each way I would guess a railcard would be a plus for a family of 4. Another place well worth a visit is Dachstein skywalk & ice caves which you can get to by bus. Hope this is of help.