Trainer Road is focused mostly on indoor workouts, you can do the workouts outside but I don’t find them to be as focused or as time-efficient. I didn’t start doing trainer road until I had a “dumb” indoor fluid trainer.
One the low end, all you really need to start is a cheap bike that can go on a trainer, compatible trainer, HR sensor, and sensors for speed / cadence. There are also some spin bikes that are compatible too (https://support.trainerroad.com/hc/en-us/articles/201682804-Compatible-Devices-and-Sensors).
FWIW - I bought the trainer used off a friend who was into cycling.
If you upgrade to a wheel-on smart trainer for about $400 (The Smart Trainer Recommendations Guide: Winter 2020-2021 | DC Rainmaker), it should be able to estimate your power more accurately and make the workouts more effective.