Salzburg to Halstatt for a day trip

It's a great day trip. Easiest way is to rent a car. Otherwise, it's a train to a ferry. Or a bus.
Salzburg has a bunch of great day trips if you use it as a place to stay.

On the other hand having a car in Hallstatt can be a burden. I speak from the experience of driving into town and seeing cars parked all along the road then in the centre one of the car parks was closed (the second one), the first one one was not only full but there was a three block long queue of cars waiting to enter. The third car park was a very long walk from town and it had a queue too. I ended up driving all the way around the bottom of the lake and parking at a train station, taking the train one stop to the Hallstatt station and taking the ferry across. At least after all that the ferry dropped us off in the centre of town and that horrific walk was not needed.

So it may vary from person to person.

Thank you... How long of a car ride is it from Salzburg to Halstatt?

It is 50 miles, a 1:30 hours drive at best, if you do not stop on the way. The route is very scenic, passing Lake Fuschl and Lake Wolfgang, as well as the famous town Bad Ischl (the emperor's summer residence).

We drove to Halstatt from Mondsee, near Salzburg. (Similar driving time; per G-map, 1.25 hr from Salzburg.) Getting an early start (this was September 2017) may have helped (arriving between 9:00-10:00 a.m.) because we were able to park in the large lot at the south edge of the village (Parkplatz P1), just beyond the tunnel leading into town. It's definitely worth the effort. We particularly enjoyed riding the little ferry boat ("Stefanie") across the lake to the RR station, and back -- getting some great photos from the water. Make time to take the funicular up to the Skywalk for a terrific birds' eye view of the town and the surrounding area (and perhaps enjoy a beverage, snack or lunch at the gasthaus up there). The drive in is beautiful; there's a handy pull-off on your left as you approach the town with additional nice views. (Not sure if you're driving; if not, arrange for the earliest available bus or rail arrival time.)

I wouldn't rent a car; I'd rather ride in a bus and watch the scenery. The 150 bus leaves from in front of the Hbf and takes 1½ hours to Bad Ischl, from where you take a train to the Hallstatt Bhf, across the lake from town. The 150 bus goes through St. Gilgen and alongside Wolfgangsee. Total trip time, by bus and train, just over 2 hours.

Will we be able to see most of the views in Halstatt?

Hallstatt is very tiny. You can easily see the entire town. It's about a 1 km walk along the edge of the lake to Lahn, from which the incline goes up to Rudolfsturm, for a wonderful panorama view of the lake and down into the town. However, I would suggest spending a night just to more completely enjoy the beauty of the place.

How do you buy tickets for the 150 Bus and the train to Hallstat? Can you buy them the day of travel?

I guess you get the bus tickets from the bus driver, and the train tickets from a vending machine at the train station.
(Maybe there is a vending machine for bus tickets in Salzburg at the main train station bus stop.)

you get the bus tickets from the bus driver, and the train tickets
from a vending machine at the train station.

That's almost what I did, but my accommodations were in Lahn, which is where the bus station is. I took the bus on from Bad Ischl to Hallstatt Lahn and saved the boat trip and the 1 km walk along the lake to Lahn. Alternatively, I could have taken the train from Bad Ischl to Obertraun, one stop beyond Hallstatt Bhf, and the bus around the south end of the lake from Obertraun to Lahn.

I bought all of my tickets from the bus drivers.

A day trip from Salzburg will allow a brief look at Hallstatt, but if you're planning to also go to Rudolsturm and take the Salt Mine tour, that might be pushing it.

If you'd rather not worry about train or bus schedules or renting a car, you could also take a day tour - http://www.bobstours.com/hst.php .

The Salt Mines Tour is pretty incredible and very worth while, but then I'd advise spending the night, and not being on such a tight schedule..