A year ago I started cycling indoors on Zwift with the Wahoo Kickr Snap smart bike trainer. I had some of the other accessory gear choices such as a fan (must have), cadence sensor, and heart rate monitor. I even setup a smart TV to work with Zwift or show movies, but there are 3 specific pieces of gear I wish I had known about and had a year ago when I got started.
1) A trainer tire: if you are using a wheel on trainer this will cut down on noise and wear on your tire
2) Crank Arm Power-meter instead of a smart bike trainer: I use my bike outdoors as often as I ride indoors and I could have spent less money by buying a "dumb" trainer and investing what was left over into a power meter so that I could have power data both indoors and outdoors.
3) A DIY standing desk. The desks they sell for indoor cycling were way above my budget for a desk. I was able to build an even better, adjustable desk using a Bike repair stand and some scrap wood/pipe.