I work at a local Lexus dealership & from my experience there are less than a 1% failure rate on the 4.0 and 4.3 liter V8's especially if they are maintained, most of the time even if there not. only failure Ive seen in 4 years is the original timing belt shredding on a 250,000+ mile engine and bending the valves on one head. that belt had been on borrowed time for a loooooong time.
With no indications of any problems i don't think you have anything to worry about assuming the timing belt has been changed. You can always run a compression test to verify your engines condition.
cost on rebuilding these engines can only be based on a true tear down and inspect. when i was working for Toyota ten years ago we had a huge recall on truck v6 headgaskets that on about one third we had to rebuild the entire engine. Most we were able to just hone the cylinder and replace rings and bearings followed up with a valve job. very few where damaged so bad we had to replaced the blocks. but it did happen. i must have rebuilt roughly 300 engines and replaced about 10 blocks. they wouldn't let us bore those. I pretty sure you cant bore the 4.0 liter but don't quote me. you know aluminum block, with thin steel sleeves doesn't give much room for oversized pistons.
I'm located in your general area if its a serious issue you want to move forward with PM me.
just my 2 cents don't buy a rebuild from a re-manufacturer for any Lexus or Toyota for that matter. you might as well be dropping in a 350 Chevy.
