re: 2001 Lexus SUV, 162K miles, $3.5K.....worth it?

I am going to assume it is an RX because if it is a GX or LX at that price is almost certainly has issues.

It is a solid vehicle and if everything functions normally it will likely go a LOT more miles. The one issue is the Lexus 3.0 engine from the era is known for sludging.

Outside of pulling the valve covers there is no good way to determine if the engine has sludge issues but if it emits oil (blue) smoke on startup or the dipstick shows black oil I would avoid it, though both are true for any car. The 1MZ-FE is an engine I would recommend being scrupulous about oil changes. 5k or 6 months and use synthetic. For most engines this is overkill but it is solid preventative maintenance for that particular engine.

BTW I owned a 1MZ-FE in my last travel car and it had 390k on the clock when the car was totaled. I followed the oil change advice above and the engine internals were never an issue.