Congratulations on getting to have your boy come home
I'll take a shot at answering some of your questions.
I don't know about cows and bulls.
Toxic plants will depend on where you live. Check out this thread for more info: Toxic Plants and Trees
A dirt paddock is unlikely to support enough green stuff to cause founder. I'd be more worried about colic from a sudden change of diet. Does he get weeds at his old place? If not, I'd block off the greenest part of his paddock and let him onto the weeds for short periods of time, gradually increasing each day.
Shelter will depend on the weather you need to protect him from. If you have cold winters you probably want a run in. If you have mild winters and hot summers you may just need a shade.