With a low cost wheel-on trainer you can use just about any old bike you have on Zwift or other online/virtual biking options. That will run you $400 - 500, usually sales happening now or after the holidays. That would give you a much better experience than on on old style (dumb) indoor trainer.
Can't help you with RGT - to me it looks like you will see videos of bike courses vs. any true type of online/virtual biking. You could go on eBay and buy used biking videos and put the stationary bike in front of a TV/DVR (VCR if you have old equipment, like that stationary trainer!)
If you do want to stick with the stationary bike, you can ride on Zwift without power measurement, but you do need speed input as a minimum. If you can get a magnet on the spinning part of the stationary bike, and mount a sensor (like the Wahoo speed sensor you mentioned), you can use that on Zwift. You could also put a cadence sensor like the RPM cadence, if you wanted to track cadence (and see your feet move on the bike on the screen!). You could also add a heart rate monitor if you want to track that. The minimum is the speed sensor and it may not be easy to mount on the stationary trainer..
Spending the money on the wheel-on trainer will mean you don't need to buy a speed sensor, so that offsets that cost a bit. You could save some money if you found a used wheel-on smart trainer. As far as the cost of Zwift, you can suspend your membership for any months you don't plan on using it. But, virtual biking is definitely a "you get what you pay for" experience - so it all depends on how much you really want to get out of it.