Congrats on your baby. My heater core started leaking slowly over the last 6 months or so. It was not a problem in the summer because my heat controls were set to full cool (which, if I'm not mistaken, cause the heater valve thingy - I'm having a brain fart here - to stop coolant flow from going through the heater core.
I researched it and saw that it was going to be a major job, so I had mine done by my indy mech for $625 including a new cabin filter. It was money well spent in my opinion because even he said it is a big pain in the butt to do.
Good luck. Also, he said that my hazard flasher button broke upon removal which is VERY common and he had to source a used one, so if you do this yourself, be very careful in removing the hazard switch.
Calypso Red 1992 525i with 183K miles
1991 735i - Sold
1992 525i - Sold
1995 325is - Sold
2000 528i - Sold