Hi David,
this is a good site, but for a slightly bigger audience go CLUBLEXUS.COM.
As to your Q's, I would suggest that with a decent LS400 running (purring) along as as well maintained one should at a steady 75 mph (cruise control on) you should get close to 28 / 30 mpg. My Mk III 1995 does.
Traction control is not standard, I do not have it, but while the car does not fly off the mark with only a small amount of right foot pressure, put the box into ECT PWR push your foot down and the baby will fly. Bear in mind this car weighs close to 2 tons and even with 260 horses doing the work it takes a lot to over come that sort of inertia. However, at almost any speed (and I do mean almost any speed) kick down and she will fly. If you are in the sports mode you can get the car to double kick down, get the revs up near the red line and you really do fly. There are not really many other cars that can stay with the LS then.
Bear in mind tho' this is not a sports car, but a car that cossets you and allow you to dive all day and still leave the right hand front door as fresh a s adaisy.
Be careful with your purchase, get a car with a full dealer service history but do not get too hung up on mileage. There are many guys on clublexus with LS400's with over 250,000 miles and some with over 300,000. How she has been looked after is the most important thing, it is reciprocal - look after her and she will look after you. Are all of the stamps coincidental with the service intervals in the service manual. Basically at least once a year or every 9000 miles. "A" service "A" service then major or "B" service. Cam Belt change at every 60 000 miles along with new plugs.
The LS400 is not over burdened with little elctronic gadgets for the driver to play with, but in reality who really needs those disrtactions, any additional connections that can go wrong and cost you money. The 7 series BMW is apparently prone to computer glitches and having an indication of you mpg and every twist and turn is not really neccessary. you will soon learn to judge how well you are doing and feel the pinch if you have to fill up the 85 litre tank too often !!!
My car is a 1995 mk III with 122,000 miles on the clock. I love the gentle tick of the engine and roar when I do want her to move, but for normal use this unit is never under any stress and should last a long time if shown repsect.