98 Dodge Ram question(s)

Take a look down the throttle body to see if any oil is puddled, that will tell you if the plenum gasket is bad. Chances are it is. Pull the IAC(idle air control) off the back of the throttle body, they get carbon build up and won't idle well.
If you replace the o2 sensors in the future use NTK, these trucks don't like Bosch.
I've been using a synthetic blend 10w 30 oil for 10 years in mine.
The transmission cooler line has a check valve near the radiator that could fail or get clogged up, I replaced the rubber hose section with the check valve with a piece of high pressure hose made for transmission coolers.
The heaters in a word, suck. If the heater core isn't clogged up the blend door sticks.
When you start driving if you get detonation or pinging under load there is a technical service bulletin for plug wire routing and going to a step cooler plug, which worked for my 98.
It sounds like you have a good plan for this sharp looking truck, best of luck with it.