How many lifetime miles should this vehicle expect

Hello, so many factors are involved with the "lifetime" mileage potential of a vehicle. Too many to even consider. Only you as the owner can weight cost / benefit and make sure you are throwing good money after bad. Consider that your vehicle is 18 years old and has safety features, economy and body hardware of 2001 compared to tremendous advances today in safety alone. Also, consider that repairing your transmission will have no bearing on the engine, differential(s), axles, anti-lock brakes, fuel system, ignition system, cooling system, exhaust, emissions components, processors, air conditioning, power window components, locks, latches, instrumentation, audio systems...the list goes on.

PROVEN FACT - the 4.3L engine is as good as it gets! .. Routinely see 300k miles without incident. Only you can decide if the rest of the vehicle is in good enough shape to invest the money into a replacement transmission. Personally - I would fix it provided everything else is in good condition.