Level of difficulty of a BSN program for an international student

It really depends on many things.

First, your English (speaking, reading, writing and understanding) should be more advanced than the level needed for daily conversations. I do not even know how to compare it with TOEFL; IMHO if you can do band 6 or up on IELTS you will be OK. Schools do not always require tests but if your home language is not English I would at least go to library and try them over books. If you have a more or less pronounced accent, it will be huge hindrance unless you live and work in very diverse place.

If you have some sort of biomedical education, the scientific component will be at least easy enough in order not to worry about it. Chemistry, physics, A&P and bio are all 90% within European high school program. The main problem is incredible cultural difference between US medical system and nursing as part of it, and those in other countries.