That's a decent deal, especially considering the known good history of the car and the $1500 savings on CA sales tax on sales within family. You should have no problem in avoiding the sales tax, check DMV website for instructions. I sold a Jeep Grand Cheroke to a family member years ago this way, no smog certificate (for me being the seller) or sales tax payment (for buyer) required.
I have 47K miles on 07 IS350, still runs like new. Tires, brake pads, battery, wiper blades are really all you need to keep an eye on in near future with this car. Check that recalls were addressed (these cars have had a few), but those will be handled by dealership at no charge so it's not an cost issue. Try to get the maintenance receipts from seller, but if Lexus dealership service performed maintenance, the information is available on-line once you register the car on Lexus website.
What color exterior and interior is the IS250?