Hey YSL
I'm a first year student at Uni Bocconi and I'm doing the BIEMF program (the bachelor taught in English).
First of all, if we're talking about Bachelor, Rotterdam School of Management hasn't an age limit. I was accepted there too!
So no, Bocconi doesn't have it. neither Uni Amsterdam I suppose!
Second of all, no, no need to convert. Half of the internationals here have IB, so the uni perfectly knows about IB!
Last, for the letter, well it doesn't have to be the same as the one for UK!
In the end Eco, Management and Finance are all extremely interlinked.
The letter should focus first of all of you as a student: introduce yourself, say what you did, what you do and where you're going.
Then of course 50% should be on WHY you want to study that subject; you could focus on one of the three areas you'd like more! Although don't be TOO specific.
I remember my motivational letter for Bocconi was completely different from my UK one; in the UK (for pure Economics) I was very specific, I wrote about my economical thoughts, expressed my view on neo-Keynesianism and all of that. Didn't have to be THAT fancy in Bocconi (because it's much more of a business school!).
Also, if you want reputation I would ditch Uni Amsterdam. Go for UK (good ones) or Bocconi. If you really want Holland go to Rotterdam School of Management, but Uni Amsterdam isn't so well known!
I would suggest you Bocconi if you can, I love it here
Contact me if you want to know more