Trainers come in a lot of flavors, features, and price points. First consider the resistance methods:
Wind: cheap, reliable, noisy, varies only with wheel speed.
Magnetic: can be variable, fairly quiet
Fluid: can be variable, quiet, smooth, some are known to leak.
Kinetic: I have no experience with these.

The variability is either the product of your selected gear rations or adjustability in the system itself. Some have a number of bells and whistles to make the trainer a bit more useful or entertaining, but aren't necessary for a decent workout.

The addition of a flywheel makes the resistance a bit smoother and realistic, but also tends to negate some of the value of the trainer by giving your legs a rest. There is little benefit to coasting.

Also, consider getting a smooth, cheap tire for the trainer (some of them wear tires quickly) and a riser block for the front wheel so it doesn't feel like your pointing downhill the whole time.

Expect to pay $150 for a new, basic, good unit, and much, much, more for a really hopped up computerized model. $300 will buy a really good unit without the electronics. The price difference may reflect the overall quality or simply the brand name.

Most people find these things remarkably boring.