I bought my first smart trainer late last year and it's the Kickcore from Wahoo. They run $799 at places like REI. I know it's above your budget but they do have 20% off sales which brings it in closer. I would wait to purchase a smart trainer from any of the big names as it sounds like you're going down that path..
You could pull the cassette off your bike and use it on the trainer to save around $70, or you would have to purchase one to put on the trainer. Some trainers may come with a cassette but it has to match the number of gears on your rear hub.
The trainer I'm using also has Watts, Cadence and Speed outputs so I didn't have to purchase additional sensors. For a training app most have a solution for the trainer supplier. I do a yearly subscription to Zwift which has been excellent and has helped me stay interested.