Okay, the engine you have in your Belta is a 2SZ-FE. What's the current mileage on your Belta? Is the transmission giving you any kind of problems? If it isn't I won't recommend you change the Transmission fluid yet.
But, if you still want to change it here are the answers to your questions:
1. What's the best replacement oil for the CVT gearbox. Is ATF type WS from Toyota the best option? How much oil would I need and what is the approximate cost?
Toyota Genuine CVT Fluid TC is the correct fluid for the vehicles with CVT. I have no idea how it differs from the ATF type WS.
2. Is there any way to determine if the gear oil is at the correct level? I have tried searching for a dip-stick or gauge of some sort but have been somewhat unsuccessful.
It doesn't appear that there is a dipstick for checking the Transmission fluid. When you remove the drain bolt (With a 6mm hex) there is a second 6mm device inside, which is the "filler tube". With the drain bolt removed, you add fluid to refill hole (Which is a bolt on the top of the CVT gearbox) until it overflows the "filler tube" in the bottom and starts to come out of the drain hole.
3. I suspect the drain nut/plug is situated at the bottom of the gearbox facing in the direction of the radiator. Please correct me if am wrong.
It is the 6mm hex bolt on the bottom of the CVT, you remove it and remove the filler tube inside of the hole to drain out the old fluid.