Hi Malcolm
The 600 was as you know a very expensive car when new and limited in numbers compared to other models. We only have a handful of members owning one - For this reason there is not many posts on faults etc.
If the selling dealer agrees, I would get Lexus to do an inspection expecially the suspension to ensure everything is ok - it is a heavy car and with the added weight of the battery pack the suspension has to cope with alot weight.
With regards to the battery: Its too old for a Lexus warranty or Hybrid Health Check - the cutoff is 10 years or 100,000 miles - this is something that you can talk to your dealer about.
If you do have a battery fault - its normally just 1/2 of the many cells and there are specialists around who will remove and dismantle the battery - check and replace the weak cells and recondition the battery for around £700 - this is better than Lexus replacing the whole battery which is around £2000 - 3000.
Hope this helps