I've test driven an X3 with 18"s, for me it's quite fine even with the run flat tires. I'm really not impresd with the Nav.systm, i've got a friend who had a big problm with his bmw nav. Ended up payin freakin lots for it to be fixd, i'd suggest that you buy yourself a TOMTOM which cost only around 300$. As for the extended waranty, i am not very familiar. But i took a 3 year/60k parts and service waranty for 500€. My waranty is different coz i had my x3 on a Diplomatic price which cuts down 30% of the price. In return my warranty is differrnt! (wise aszzs lol).
Hope this helps