@orozcom There are IOT style arduinos that have accelerometers built in or you can get them on a breakout board.
The isadora part is the "easy" part, at least once you got the OSC sending. It sounds like you need to start playing with arduinos first. There are some resources here on the forum but a lot of your questions will probably warrant a visit to the arduino forum as well.
Try the teensy. It can be set to act as a HID or midi interface so that will be very isadora friendly in terms of communication (instead of OSC). The pulse sensor will work fine with the teensy.
here is a guy setting up a accelerometer with the teensy. If you google 'arduino pedometer' there are plenty resources on how to detect steps.