What is the difference between a cheap and a more expensive submersible well pump?
Stainless steel casing.
Only if you think there is something corrosive in your water. As a general rule, a submersible pump is under water all of the time. Rust requires oxygen, so unless you constantly pump it dry and expose it, the stainless can be a waste. But it does help with other corrosives. 20 years is a long time, was that pump stainless?

Plastic vs metal impellers?
Plastic is cheaper, but ANY tiny amount of possible grit chews it up really fast.

230 v (Pump is 300 feet from house.)
Absolute minimum; don't even consider 120V.

Automatic low water shutoff?
Absolutely, unless you have some other means of doing that.

Thermal protected motor.
You are required by code to have running overload protection for the motor. Using a thermally protected motor qualifies. If you get one without it, you must add a thermal overload relay to your controller.

Is Variable Frequency Drive worth it?
Maybe. Is variable volume important to you? In a pump like this, if you pump at full volume and turn it down with a valve, you are wasting some energy in that process. Using a VFD to lower the flow accomplishes the same thing while using significantly less energy in the process. But if you are NOT varying the flow on a regular basis, then the losses in the VFD will actually cost you MORE energy in comparison. So it depends on what you want to do with the irrigation system. A side benefit to a VFD however is that you can use it to convert your single phase electric service to run a 3 phase pump. The advantage there is that the pump is more reliable in the long run, there is no "start capacitor / run winding" in the motor that must be switched in and out.