looking for very reliable luxury SUV, older ok but needs to be one that will go a long time

RX or MDX will suit your needs ... still, have them inspected by a mechanic (don't rely on seller even if it's a garage) ...

BMW will not only be less reliable, it's usually more tricky and expensive to get them repaired.

Not traditional SUVs, but suggest you also look at the last gen Volvo XC90 and the Cadillac SRX.
Both are comfortable vehicles that drive wonderfully.

The Lexus is easily the most reliable of those mentioned. The Acura MDX has had some issues here and there, though usually not something serious.

I wouldn't get a Sequoia unless you really need something that big. If you don't need a Lexus badge, the Highlander is mechanically similar to the RX.

With a BMW or Volvo that's more than six years old the odds of expensive problems are fairly high. Especially with the BMW.

The SRX is a good size but can be pricey. The Buick Encore can be purchased new online for $18,000 or less. It was compared to the first generation BMW X1 as a $13,000 less BMW by Automobile magazine.