I'd say the lower ball joints and radiator at the minimum if not done already. Even if you don't do the work yourself you can skim through @mtbtim videos to arm yourself with idea of what might be good to get done and how thing's work, it's a true gold mine of info. Timmy The Toolman - YouTube

Timing belt, water pump, thermostat, radiator, hoses, coolant.
Lower ball joints and bolts (from an official Toyota dealer).
Check brake pads and the like.
Replace brake hoses with braided stainless.
Brake fluid flush.
Power steering flush.
ATF drain and fill.
Differentials and transfer case fluids.
Spark plugs and wires.
PCV valve and grommet (be very careful the grommet doesnt disintegrate in to the hole).
Clean the throttle body and MAF.
New alternator brushes might be an idea.
Check for any leaks, including the rear axles.
Check the state of the CV boots, bushings and stuff like that.

The 4th gens look LESS reliable from the used ones I've looked at but the 5th gens are super solid. Only issue seems to be the 1GR-FE timing chain cover leak which you can ignore indefinitely because it's not pressurized which they seem to have addressed in the later years. I'm almost dissapointed by the complete lack of anything I've had to do on mine and it's about to hit 200k miles and my gf clocks a lot on miles on it. She still gets close to 20mpg out of it too. I was looking forward to learning the 5th gen platform. We'll see what the 200s bring I guess. I don't see any reason to jump ship though, especially with the market right now and what's probably coming down the line economy wise.