We bought an RX 300 at 10 years old and rust was similar to what is pictured. Once i bought it I rubbed down as much as I could and painted rust converter on the steel and then primer and finished with Bilt Hamber wax. The wax had to be applied every year otherwise it washed off, particularly on the exposed parts. Sadly, every year, the appearance got worse - and I wasn't doing a huge mileage. At 5 years ownership the rust looked to be becoming a problem, coupled with water leaks into the interior and I decided to move it on - for a Seat Ateca 190 tdi (wife's car). I do the same routine with it, and it's so much better protected from rust than the Lexus (it is a lot newer to be fair). I've checked on the continued MOT history of the RX - and my daughter in law's one of the same age - traded in at the same time and for similar reasons. They both have had sills welded and reports of the structure surrounding the subframe compromised - i doubt they'll last much longer.
To cut a long story short, try and buy the best condition one you can and look at numerous examples, especially if you hope to keep it for some time. Perhaps get a proper underbody condition survey and work out if rust being sorted long term is viable prospect. Cars in Scotland do get a hard time though, especially by the sea.
As far a engine noise goes I can't really comment, but our previous models were very quiet, and i'd expect nothing less from a lexus. I do wish they'd properly seal the body and underneath though - it's not rocket science.
Good luck.
Pete