Yours must be the most reliable X5 modification to this time.The engine is bulletproof and if a short list of maintenance items are taken care of nothing bad will happen.I have seen many of those M54 engines with over 200 k miles in excellent health.Even if you loose the engine replacing with a good used one with history will be $3-4k.The tranny will inevitably die one of these days,but that is nothing $1-2k wont fix reliably.
Now,getting into E70 could be a big ticket.Things are infinitely more complicated and problems much often.Price if the used parts and engines,transmission is times higher.As someone mentioned pretty much everything before 2010 will turn out a money pit,except if bought for amazing bargain.
I had a '03 4.6is with 130 k miles and now '12 35D with 33k miles..I love both cars and either one has it's upsides and downsides,but reliability is neither's strong side.I have never been stranded on the side of the road,but every week there was some [small] issue to adress with either one of them.I can imagine that my X5D with 130 k miles will be a bigger reliability than the 4.6is,but we'll see.