I bought a 1997 LS400 with 137k on it and kept it for a year(ish) before selling it to the parents. They've now owned it for around a year and it's up to 16X,XXX.
Before buying it, I did a compression test on it to make sure then engine was still in good mechanical shape. It passed with ease.
I then, upon taking ownership, did valve cover gaskets, plugs, plug wires, air filter, etc. General maintenance.
It ran beautifully. I would drain and fill the transmission with every oil change to give the trans an easy "flush" so as not to force deposits elsewhere in it.
No issues for a long time, save for a weeping pan gasket (which is a PITA to change as it drops down on both sides of the crossmember) and strut rod bushings (have to be drilled/pressed/burned out and have the new ones pressed in.)
I changed the power steering pump out for a re-manned one as well as re-o-ringed the high pressure line. While you're in there, remove the alternator and have it rebuilt. When the power steering pump leaks, it leaks right on to the alternator. Money well spent as it's 15+ years old and probably one it's way out.
Parents did just recently have an issue with it, though. A, not-so-rare, issue with the ECU. Leaking capacitors. It is currently in the shop awaiting diagnosis, but the tech believes it could have washed three cylinders...
Some issues I would look at before buying;
- Oil pan gasket
- Power Steering pump and high pressure line
- Brittle vacuum lines - Causes vacuum leaks
- Strut rod bushings - Cause a shimmy/vibration at speed
- Leaking ECU capacitors - Can cause any and all ECU related issues
Other than that, I loved every minute of driving it. 20+ MPG, quiet, reliable, smooth... Would highly recommend a clean one, if you can find it. They pop up occasionally on craigslist.