If I was contemplating purchase of a car approaching 60k, I'd insist on the spark plugs being done as part of the deal. Difficult to check though !
I use Lexus Cardiff for my servicing, it's a condition of the Lexus warranty. I only do 8,000 a year so not had this one done there yet, but I purchased my warranty from them and they attended to my headlamp issue.
Can't comment on power band. True to say, the IS250 is not a particularly quick car, but adequate in my view. A lot of fun can be had with the 6 speed auto using the paddles in manual mode. It can shift a bit !!
As Bob says, inspect the exhaust. Mine still has paint on it, I suppose it depends if it's been driven on salt laden roads in the winter.
The Lexus IS "works"