from what i remember reading a long time ago, battery drainage is notorious in the X5 if car is not started often (ex: not started for a few days etc.) so it might be normal for you to get the reading of lower voltage

i don't know much about the specs of the bosch battery, but it should at least have same specs as oem

here's specs of most common aftermarket battery for X5 and 3 series bmws in the US,
Duralast 49-DL perfect fit (don't go by store computer, they don't have it listed under X5 or 3 series) better than OEM, more powerful
and only $65

Duralast 49-DL
Specs:
Cranking amps 1000
Cold cranking amps 850
Reserve capacity minutes 150
Vent hose included