A lot of the times your school might administer a required or optional NBME. (Sometimes schools use this as like an entrance exam into the clinical years). I ended up taking that one as my baseline exam because it was free and it was an NBME you can't buy (it is only given to medical school administration faculty to give to students).
So, talk to your school to see if they have anything like that. If not, I don't really think it matters what an NBME you purchase to do first.