Hi Jim,
The 2006 model's do have a few recalls/tsbs that should be addressed if you do go ahead with the purchase, you can find out which are outstanding by entering the VIN into the Lexus website or contacting your local dealer. The only other things I can think of specific to that year would be to check under the floor mats for moister, as there was a problem with the A/C drain becoming clogged and leaking water into the cabin.
Of course, the usual used car caveats apply here as well, especially since this car is past its factory warranty period. Make sure the tires are good, and check to see if they are summer only - if so, and you are planning on driving it in the winter, plan on getting a set of snow tires. Check the brake pads, as brake jobs on these cars can be expensive, unless you are a DIY'er.
We put 80,000 miles on our 07 IS350 before getting rid of it a couple of months ago and it was trouble free.
Good luck with the search.