Regarding the trainer: Call around to local bike shops to see if they will let you try out their smart trainers. I know at least 1 local bike shop near me that sets up a smart trainer for customers to trial both Zwift and the trainer.
Regarding the bike: My Wahoo Kickr uses a Shimano/Sram 11 speed freehub, which means it could run anywhere from an 8 speed to 11 speed cassette. Which means it is compatible with pretty much any cheap bike you might find at Walmart or Dicks or second-hand on craigslist or ebay, along with high end super nice road bikes. Their website says that it’s also compatible with mountain bikes with the use of an adapter. So depending on what your goals are, you may not need to spend much money on the bike you plan to use with your trainer. And the mountain bike you already have just might work. Look around a bit to see if a shop will let you fit your bike to one of their trainers for a test spin.
Regarding my experience: I only have experience with my 1st gen Kickr. I bought it second-hand 3 months ago for far less than full retail and have been very pleased with it.