Hi Dave,
I note that you are in Lincolnshire. I'm in Surrey, and I think the market may be different - I found that GS300's that I looked at were owned by mature people who purchased them for their own use - They were generally very well looked after, and were often on their first owner. I think I looked at 6 GS430's. Not one of them had been cared for, and all but one had multiple issues. On the one that didn't have multiple issues someone had tried to hide massive rust in the sunroof mounting. I found that all of them had companies as first owners - So they were being used by people who didn't have any incentive to look after them. Typically after two years they were sold to people who couldn't really afford to run them, and from that point on they went downhill rapidly. Maybe there were fewer sold as company cars further away from London. If that's so, the new price of a GS430 would ensure that any private first owner had enough money to maintain the car in good condition, and keep it longer.
At the time I couldn't find a good GS430 as far afield as Oxfordshire, which is why in the end I gave up. Clearly there are some around that have been cherished, but I couldn't find one.
I've just hit the same problem with LS430's. Last Saturday the 2001 LS430 I bought in 2011 was hit side on by a vehicle travelling at high speed. Result - Write off. I looked around for another LS430 (because, other than possibly an LS460, I can't think of any other car I would want). Again in Surrey and the home counties I visited a "lovelley" car in excellent condition that had two broken wing mirror mounts, a steering column with failed motors (£2004.30 to be rectified by a Lexus main dealer) and dead rear amplifiers. Someone had tried to get the sound working, messed up, and done a bad job of putting it all back together again (As I write I think it's still being advertised). Another car looked great on Autotrader, but in reality it was fit for scrap and nothing else.
I was just about to give up when I found one of those cars from fables. 2001 with 92,000 miles, always serviced at Lexus main dealers and since 2012 it had only covered 1000 miles. The elderly owner had kept it in immaculate condition, but had switched to a Mini in 2012. I checked it very very carefully including talking to Lexus Bracknell about its history, and I now have a car that should last me 5 - 10 years (unless I meet another idiot).
One final thought - It's probably a good thing that I didn't get a GS430 because, like the BMW 3 Series, they were so good to drive that they brought out the hooligan in me. Driving an old man's car instead has probably saved my licence...