Hi there! I am a leonberger owner, and I can tell you they are wonderful dogs

Yes they are very big, and very strong! But they are very loving. We have 3 small children and our Leo is super patient and sweet with them.
Pretty easy to train, but there is a lot of hair keep in mind. Hair everywhere, and will be everywhere all the time. You'll have water all over your floor because they never keep their mouths closed after drinking! Things will never feel very "clean" at home

She is expensive, we live in Sweden but I am sure the cost will be about the same anywhere. They are slow growers and take 3 years to get to their full stature, so your Leo will be on puppy food for quite awhile.
In general, it is my favorite dog breed. She is very attached to her family, kind, gentle, patient and attentive. So long as you have the time, space, and finances it is a wonderful dog breed!