Congrats on the "upgrade"
As far as safety is concerned there are lots of good baby buckets out there. I've found that not all of them fit well in the X5 though. If you go with the latch system you need to be concerned with the curved cushion portion of the seat near the doors. The base of some kid seats don't clear the cushion so don't line up properly with the latch anchors.
The safest place for baby is the center seat but you can't use the latch system there so you give up some convenience. Small price to pay IMO.
Measure the clearance and go shopping with a tape measure.
G/L