Hi There.
In terms of keys, make sure you have at least one master key. If you don't, then you will have to replace the ECU to accept new master keys and this was over £3,000 if I remember rightly. So you should have two master keys and a valet key. The valet key will not work the smart locking and also won't open the boot.
As far as I am aware there is no radio code for the LS430. I do have the newer version though so it might be worth checking for the older version.
You should have a locking wheel nut with the car. I belive these were a standard fot from Lexus. Mine came with a black box which had four regular wheel nuts and a locking key. The locking bolts had already been fitted to the wheels.
It should come with a service book and a full service history. If it doesn't have a full service history then walk away. These may be reliable but I'd hate to think of the size of the bill if a new engine was required due to lack of maintenance. Also, press every single button and make sure it all works.
The drivers handbook is very comprehensive and covers everything. There should also be a second book for the navigation and stereo, aircon etc. It is worth having this because some of the things, like setting up the bluetooth are not that easy sometimes.
The SatNav is DVD based and is all on one disc. The DVD drive is located in the boot on the drivers side. The likelyhood is it will have the same disc from new and will need updating. You can do this through Lexus Map Updates website and I believe is preety reasonable.
So long as you have a full service history, that is traceable and checkable with the dealers that have done the work, and everything works, then you should be onto a good one. It is also worth remembering that cambelts need changing after 10 years or 100,000 miles.
Hope this helps,
Jim.