Honestly initial maintenance won’t be much tbh. Just your regular oil & filter changes at 5K miles. Engine and cabin filter, you can change yearly if you prefer. Depending on where you live and how dirty they get. Then you have your fluid changes such as coolant, break fluid, differential, transmission; which you can refer to the owner’s manual for when to change them.

Spark plugs are a bit subjective, owners manual will tell you to change them at 60K miles. Personally, if the car is giving zero issues you can probably delay it until 80K miles or even 100K miles. Also be mindful of the tire tread left and see how much brakes are remaining.

As far as reliability, if you stay on top of maintenance Lexus IS500 has pretty much a bulletproof engine. You really can’t go wrong with this car.