A well maintained x3 will last a very long time:
One super commuter got 208k trouble free miles out of his 05 (his reports above). I have 109k on my 09 and it has been trouble free.
With your cars age the known maintenance areas are:
1) vanos hoses and o rings
2) valve cover gasket leaking
4) oil filter housing gasket
6) new shocks and struts
Outside of those just routine maintenance is required...fluids, spark plugs, filters, and o2 sensors.
If you are up to date then you have nothing to worry about being stranded on a long trip.
Where I side with your husband on though is that you have to be willing to accept with an aging car non drivability things are going to break (ie window regulators, leaking vapor barriers, sun roof cassettes, cup holders etc). If your are fine with that....in the long run it will be cheaper to keep an aging paid off car than making $400 a month payments on a new one. That way you can set aside the monthly car payments for repairs.
We will be here to help too..
As a side note though...the new f25 x3 has its share of problems:
Erratic shifting and hesitation
Rear hatch wiring issues
Ecu replacement as a result of losing dsc programming.
Various rattles and premature brake wear.
Those are the notes I have in my parents 01/12 build 2012 x3 with 6,000 miles.