If you are mechanically inclined and have the ability and tools to work on a late model BMW, I would say keep it and forget the warrantee. there is nothing magic about 100K - it's just a number. You know the history of your X, and it has been relatively trouble free, so why not keep it? It's a great vehicle and better to drive than anything you will replace it for similar money that may or may not end being more reliable than the BMW. Alternator is one weak spot and an expensive one, and that is fixed. If you haven't done the cooling system, plan on replacing it sometime in the next 10K miles or so. Water pump, expansion tank at a minimum and probably not a bad idea to replace the radiator and swap hoses while you are are at it. The water pump was fixed on mine under CPO with 85K or so on it and also replaced the expansion tank and a few of the hoses. Radiator checked out so they did not replace it. I plan to replace that sometime before getting to 120K for sure along with all of the hoses. I will do it myself. It will not be a very fun project, but no way I am going to pay a shop $110 / hour and have them charge me for 150% of the time spent on it, plus jacked up parts costs.

You may consider swapping the trans fluid right now. I decided to let mine ride per BMW's original lifetime fluid policy rather than tempt fate with a high miles swap. If I had the car at 75-80K I would have done it then for sure. Swap the transfer case and both diff fluids at 100K. Flush the brake system with new fluid if you haven't done that recently.

Other than that, enjoy it - just keep up on some of the preventative maint items and you should be OK. Yes - things can break, and they can be expensive, but that is true for all brands of vehicles, and at the end of the day driving what you like is more important than potentially saving a few $K on repairs somewhere down the line.

__________________
2003 4.6is - SOLD
2003 M5 Imola
2008 Mini Cooper S
2011 Mazda CX9