LIM gaskets, potential transmission issues, hub bearings (mostly front), Coolant elbows (3.8), electric window regulators, front lower control arm front bushings.
The 3800 is much more desirable, the stuff i mentioned is common and easy to repair except the transmission. The question is not will it fail but when, it will fail for sure, plan on putting one in.
There is no helping a pre late 2003/04 4t65E, the 4th gear input shaft is not hardened and wears away more every time it goes into forth, one day it just stops going into forth gear.
No amount of fluid changes or preventive maintenance can prolong its life, it will eventually fail.
The transmissions other issues can be help by preventive maintenance but to what end? Over 100K i throw the unit out at the first sign of trouble and replace it with a bulletproof replacement from the aftermarket.
The rest of the car is great, good electrics, body, A/C, heating, rust is minimal brakes are fine.
Use the transmission issue as leverage, you need $2500 to replace it with a quality unit installed by a decent installer. If you own this car right and it has the 3800 in it there are plenty more years in this car.
If it has the 3.4 walk away, its risky without a known history.