Sam, this is a sad story to read, and I totally understand the thought of getting rid of the car.
I had similar dilemma last year, but my LS had done 230K+ and needed about 3K spent on it, to keep it on the road, but decided I did not have the skills to do it so it went to the great Lexus resting place in the sky.
Since then I have regretted it at times and then at other times decided it was the best thing to do.
As a Lexus owner of 15 years, my Merc seems so rough and ready and though cheaper to repair I spend a lot of time thinking about the Lexus I will buy when I feel the Merc is just not making me feel like driving much.
So whatever you end up driving it will not be the same as an LS400 (I did not want a 430, was not sure on a 460 though was erring on buying a ES300H before bargain - or so I thought - E350 dropped into my lap)
Best of luck with whatever you end of driving