For what it's worth the only regret I have for buying an X5 for our family of 5 is that I didn't buy one years ago. I'm 197cm, wife is 180, 16, 13 and 9yo are 180, 178 and 148. We struggled along in the e39 because I hate the idea of a Chelsea tractor. But we were just too big.
I've done 6's, v8's and v12's, saloons and tourings, in e32's, e38's and e39's and in the end bought an e53 3.0l petrol X5 and I don't regret the decision at all. I have the 535is if I want to go for a fang, but when we are 5 up, filled to the brim to go away I want security and cruising ability. The X5 does it without breaking a sweat.
As for tyres, you just need to know where to look. A got four brand new Continental CrossContact UHP in 18" for $800 all up. We've done about 30k mainly motorway driving and they still have 3-4 mm. I've thrown it over the Paekok hill plenty of times and never had any trouble in the corners, they have been excellent.
It was serviced for the first time in our 18 months of ownership by Mike Page in Kapiti on Thursday. Full inspection II was just under 1k. Including new plugs, the odd belt, brake fluid, oil, all filters etc. I didn't think that was too bad.
My budget was much lower than yours but the 3.0l petrol was still 2/3rds of the price of a diesel and they'd all done twice the mileage. Maybe I was missing in my sums something but I couldn't justify the initial price difference.
I've just remembered your comments about the 3.0 being underpowered and thirty. We average 11.2 l/100k. I don't think that's bad at all, we don't drive it around town, but I don't remember getting anywhere near that my m62 e38. Perhaps the newer petrols you are looking at are far better. Cruising on the open road is easy as is overtaking. I appreciate your friend regretting it, but not me.
I would have loved an e70 but given how cheap the e53 was, it meant I could keep the 535is for the weekends.