I used to use OneNote before I decided to see how well I could get by without the Microsoft Suite — easily, as it turns out.
There are several apps out there that are good for note taking, annotations, handwritten notes with and without stylus, etc. Some are more minimalist, like Penultimate, but have awesome response for handwriting, others are more complex and allow image, screenshot, and pdf annotation, but are sometimes not so good at handwriting response, or come with a hefty price point (for an app). Do a search on note taking, whiteboard, or annotation apps, and you will get a good list.
There is also an app called MobileNoter which claims to two-way sync with OneNote and display notebooks exactly, but customer reviews are unhappy with the editing capabilities on existing notes. Could be a good option though, with updates. It says it’s free, but there may be a monthly subscription required to sync with OneNote.
That’s all I have for you off the top of my head. I don’t thing you will find a single app as powerful as OneNote, but there are definitely a few things out there that might make it worth your while to get an iPad.
Hope that helps!