If you want a modern treadmill with all of the bells and whistles to use along with your Apple Fitness Plus subscription, I suggest the Sunny Health & Fitness SF-T7909. This is a reasonably priced treadmill with modern features such as Bluetooth connectivity, built-in speakers, a multi-functional performance monitor, a pulse monitor, a device holder, and 12 auto-incline levels.
If you are on a budget, the XTERRA Fitness T150 will help you get a good workout with an LCD screen, 12 preset options, and manual incline settings. If you want something different, take a look at the UREVO 2-in-1 Under Desk Treadmill. You cannot only do your Apple Fitness+ workouts, but you can use it under your desk while you work.
If you are in the market for an Apple Watch to use with Apple Fitness Plus, check out our choices for the best Apple Watch. If your new treadmill does not include a stand for your iPad, check out our favorite iPad stands.