Well the 3.0 is the most reliable engine for the most part. You already know all the E46 issues:

-CCV (make sure updated to cold weather version and replace dipstick guide tube to Cold Climate), in CA this isn't as important, but I would make sure to replace it, mine was original when replaced this year at 11 years of age
-VCG / Vanos - At your mileage again probably not necessary but an easy DIY
-DISA Rebuild - Easy DIY with the G.A.S. kit
-At the age of the vehicle, expansion tank should have been replaced at least once, coolant flushed twice, new hoses, Tstat, and perhaps water pump. If these are all original I would do this AT ONCE
-At the age I would inspect/replace tensioners, belts, and perhaps fan clutch (mine was almost seized at 10 years old) - All E53
-Check tire age and condition (all E53)
-Due to age many bushings/ball joints at least in the front suspension will be fairly shot, recommend replacing the front end stuff first as that is most noticeable
(This is across all E53)

Other than that just change all the fluids if you buy it, give it a good hot oil change and drive the piss out of it. I have no remarks on AT issues because I have a 5 Speed

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards