The only way I know to be sure which way to go is to disassemble your existing engine and prepare a cost to repair estimate. You can get some perspective of the shape your engine is in from compression and leakdown tests.

I suggest you will be more comfortable with rebuilding your engine as you know what it has been through and you should be comfortable that there should be not major work necessary over the next 5 years.

Unless the used engine is torn down you won't be sure of the condition and you won't know if it received TLC or was abused. You can have the same tests done plus a pressurized smoke test to get some idea. If I decided to go with a used engine it would have a warranty of some type and I would want to run it on an engine stand to cycle it to operating temp a couple of times and rev it to see if there are engine sounds that mean trouble lurks. It is likely the used engine won't be a plug and play swap. There will likely added costs.

Odds are the used engine will cost the least now but cost more to maintain over the 5 years. I don't know a way to determine which route the overall cost will be less.

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