A DIY setup can work very well, even better IMHO than the Peloton bike. From cheap to expensive:

1. Just ride by feel. You will still get a good workout. (free with your existing equipment)
2. Get a cadence sensor from Wahoo or Polar (~ $50)
3. Get power meter pedals (pedals also work as a cadence sensor), Favero Assioma Uno is a great option (~$450)
4. For the price of the Bike+, you can get either an indoor trainer with an actual bike or a high-end indoor bike with an accurate power meter. Either way you get better accuracy and connectivity to any training app.

With a DIY setup, the only thing you would be missing out on is the leaderboard, but since power numbers are unreliable on Peloton bikes (-/+10% even on the Bike+), the leaderboard is actually pretty meaningless.