Ipad and iphone work fine and also Jeff is right on, you really don't need lots of computer power for Zwift. I have run it on a Lenovo X131e, which is very basic, just one or two steps above a chrome book.
Also, to echo Nate, try it first before making any large expenditures on gear. Some people like the group rides, some like workouts, others like the "racing" aspect, however, it may not grab you. I do have to say, a smart trainer with variable resistance does make a huge difference on the integration of the game. The whole simulation is based on your power to weight ratio so how smooth and realistic would depend on how well the power curve of a "dumb" trainer has been estimated (by Zwift), or how good the power is estimated by the smart trainer or a power meter on your bike, connected to Zwift on the phone, ipad, or computer.