no worries - i just wasn't sure what you are looking for.
the answer is "it depends". i wanted tire size for capability; that lead to lift and lockers. which lead to hydro steer. MF just sux at power, so that lead to Cummins. power to go fast but whoa slow lead to disc brakes.
if you don't want tires, or power steering, reliability, power, or etc., then you don't need to upgrade anything. if you're unhappy with a particular performance characteristic (as i was), then find a solution to resolve it. like you said; happy with 45-50 mph? cool. i'm not. it's a hazard on the highway; so i changed it.
i don't remember how much i have invested, but i feel i have a good end product, and i'm happy with that. i also feel that the truck meets and exceeds in all areas of improvement that i was going after. i feel that there are enough serious upgrade threads on here that can help provide solutions to any of these 'areas of opportunity.'
it isn't done, though. i feel the need to improve the suspension for ride and off-road performance. and as that gets me farther, axle shafts and driveline enhancements will likely be next. so if there isn't a good thread showing an actual engineered multi-link suspension by the time i'm ready to upgrade, then i'll start it. promise.