How to purchase tickets and what trains to get on from Luzern to Wengen

You need the SBB app for Swiss Trains. Just go to ticket window, tell then you have Half Fare Card, and they will sell you the correct tix. WHen ticket checker comes around on the train you'll show your ticket as well as your half fare card. Yes, of course, you can buy tix on your SBB app if you want. Just choose the option that says you have a HFC.

SBB will tell you each train connection, time, platform along the way.

Swiss trains don't sell out and they have frequent departures, so you don't need to buy them in advance. However, you could use your SBB app (is that what you meant by ZVV?) to buy them. It's a handy app to have because you can also look up timetables.

There aren't any "best" trains; they're all good. 1st class has 3 seats across while 2nd class has 4. 1st class also has fewer people.

No need for stress, trains, and Switzerland to be in the same sentence. All the staff at the stations speak English and will be happy to point you to the correct platform.

ZVV (Zürcher Verkehrsverbund) is the Zürich area transport authority. Dump that and download the SBB app (called "SBB mobile", website here: https://www.micro.sbb.ch/mobile/en/home.html ).
That gives all trains and buses in Switzerland (including the ZVV area) and is available in English.

You can also purchase tickets with the SBB app (make sure your phone battery does not run out when you are on the train!), or at the stations by ticket machine or manned ticket window.

Luzern to Wengen is via the Brünig pass and Interlaken, and is a scenic ride. Everything past Interlaken is totally scenic.

Just go to the SBB counter at the Luzern Bahnhof on the second level. Ticket agents are very helpful.

There are two similarly named stations in Interlaken. Interlaken Ost is the one you need in order to change to the trains up the hill, and if you come over from Luzern via Brünig it will be the first one that you reach. In fact, since the narrow gauge from Luzern ends there you can't go further, and the automatic PA will make that clear.

If you don't intend to dawdle in Interlaken Ost or near the station you need to get a move on in order to make the connection, it is fairly close. Or you could dawdle and take the next train up the hill. Opposite the Interlaken Ost station is a 2 storey Coop supermarket with a good selection of both prepared salads and sandwiches, and all the fixings for making your own. They also have a hot and cold food restaurant.

Interlaken West is the first Interlaken station you get to if you are arriving on the main line from Bern, Geneva, Basel, etc. and is closer to the centre of the town (and across from the McDonalds). All trains which stop at Interlaken West also go the 3 or 4 minutes further to terminate at Interlaken Ost, where you get the little train up the hill.

The train that you get onto is actually two trains joined in the middle. One part is for Lauterbrunnen and is the part that you need; the other part will split off part way along and go off to Grindelwald. They are clearly marked on the outside of every carriage. Be sure to get in the right bit.