child safety seats
This is part of my day job. Most rear-facing seats should fit OK in the back of an E9 with lap belts only -- but it will be a pain (as with most 2-door cars) to secure the child in the seat, and the seat into the vehicle, back there. Seats with a removable base may be easier. Be sure to follow the seat manufacturer's instructions. Britax makes good seats; so do others. Most retailers will let you test-fit a seat in your car before you buy it.
BTW, the usual recommendation is that seat belts (or at least the webbing) should be replaced after 15-20 years -- and some say much sooner, depending upon exposure to sunlight, etc. Race belts every 2 years. Of course, any belts showing fraying or that have been in a crash should be replaced.