I think both, if it ain't broke don't fix it and if it ain't broke it will soon, apply depending on what you tackle. As an example if there is no intake leak I would use don't fix it. Reason is that sometimes one makes a mistake in the fix or breaks something else in the process that is costly repair or extends downtime significantly.

Before tearing anything down--- I would do a smoke test, pressurized cooing system test, compression and leakdown test. I would also clean the engine and verify the source of the oil leak(s). I would pull the oil pan and check to see if there is anything there that shouldn't be. I would certainly check for trouble codes. I would check for brake fluid leaks and see if air conditioning was at the right temp at all vents. The reason is that you may find other problems that are more critical and costly to the point that a prioritized budget is in order. If budget becomes a concern focus on the parts that you would have to do the same big teardown to get to if/when they die. If you don't have the maintenance and repair history I would take a shot at trying to dig it up as you could be do PM on something that has already been done.

In most cases it is best to use OE or OEM parts. In some cases there are upgrades from original suppliers or aftermarket sources that are better. None that I know of come from China.

Some of the preventative maintenance suggested doesn't necessarily fail at miles mentioned. Where your X5 has lived has a lot to do with when parts like the expansion tank, plastic parts getting brittle and coolant hoses fail. For instance, my current BMWs have been in Texas all of their lives so while it gets hot it seldom gets below freezing. The preventative maintenance I did when I lived in Michigan was 20-40,000 miles sooner than here in Texas.

I don't replace things like O2 sensors, rebuild the alternator, replace the power steering reservoir unless there is a problem. Way too much variance as to when failure can occur. Might be 75,000 miles might be twice that and the vast majority of time there are obvious symptoms beforehand.

I would start using fuel with Techron or use a bottle per tank rather than pressure cleaning, unless it is clear you have an injector issue. `

There is always risk cleaning an alternator but I have never ruined an alternator by doing so. I have used Gunk engine cleaner. Currently I use Simple Green. Good practice to disconnect the battery but I don't. I just spray it liberally and hose it off. Sometimes I do it twice depending on how bad it looks. I always wait until I think everything is totally dry and march on. I would do it before any teardown so I could run the engine and see if it was damaged.

Threads like these usually end up with posts that at least indirectly implying one has made a terrible decision or if one makes a list of everything suggested they go into a terminal case of buyer's remorse. Keep in mind that all input is anecdotal, void of the associated circumstances, that human nature tends to better at worst case scenarios and we seldom hear from those that have parts that have not failed failed or failed well after the mileage that is usually mentioned in these threads.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas