Hi,
Playing devils advocate again, the cambelt was changed way too late at 120,000 miles, should have been 100,000. Not necessarily an issue but indicative of how the car was maintained, will bet next months wages that servicing at that time was not by Lexus as anyone willing to pay Lexus prices would not skimp on leaving cambelt change. I suspect that documentation will show an ownership change around the 90-100k miles. If car was sold by a garage at that time they probably arranged the cambelt change to be able to sell the car.
Bills would be something that would be handy to see to check what has been done ove rhte years. The boot seals and struts as you say should be an inexpensive fix. The cambelt and aircon are unlikely to be, you could end up investing 50% of your purchase price again on cambelt alone so be prepared for that.
I would suspect that for the money you are budgeting there will be better examples available. The problem is that an LS400 even after 200,000 miles is still a better car than most anything else on the road within equivalent price bracket and if you havent experienced them before for comparison it is easy to be overwhelmed by the spec, quality and comfort and therefore overlook the negatives.
For what you are paying they are one hell of a car and if you are happy to risk the potential bills associated with a high mileage car then you are unlikely to find a better car for your money so long as you get a good one.
Gut feeling on this one is I would be walking away with the aircon and cambelt issues alone but thats just me. As mentioned before I dont like high mileage cars whatever the make but I like to keep my cars as close to showroom as possible and that just isnt possible when someone else has done 200,000 miles in it so I am probably more sceptical, picky and paranoid than most. Perhaps someone with a 400 with equivalent miles could give you a better insight as my 98 mk 4 has only done 55k.
To reiterate, the 400 is an easy car to fall in love with just dont let your heart rule your head, £1600 isnt a lot of money for a car but if it's all you have got then its a lot to lose and if for whatever reason you have to try to sell it on needing a cambelt and with no working aircon no-one is going to give you anything like the amount you are being asked to pay for this one.
With that mileage and the issues you are already aware of I would say its maybe worth 50% of the current asking price on a good day.
Have a shop around to see what other examples are available there are good ones out there and the right one is worth waiting for.
Just so you know, there is probably no-one worse than me for being "I want one and I want it now" so understand the temptations but I have always ended up regreting those hasty decisions and literaly lost tnousands as a result so now tend to over compensate.
Whatever you decide to do I hope you get sorted and enjoy what is still one of the best cars ever produced.