Congrats and good luck with the new additions to your family.
I have a 2 year old, a newborn, and an '03 525. The Britax Roundabout fits just fine (forward facing) with room to spare even when the front seat is all the way back. For the newborn, our base and rear facing seat fit fine as well with the front seat almost all the way back.
The one thing I can't tell you about is the LATCH system. For some reason, I can't find the anchors. Drives me nuts. I just use the seat belts to hold the child seats in place. When you get your 5er, I suggest that you have your sales assoc specifically show you where the anchors are. I thought it would be really obvious, since there were flags showing the location for one anchor on each side of the backseat. I must really be dim, b/c I can't find the anchor even where the flags are.
Good luck and let me know how the LATCH system is working for you.