1996 Nissan Maxima High Mileage Driver Needs Tips

Hey all -- Any advice you have would be amazing!
I have recently taken a job driving approximately 450 miles per night in my own vehicle.
I am considering 2 options - either buying a car specifically for the job or renting a car with unlimited mileage.
 
Buying a car I would most likely buy a Honda or some kind of 4 cylinder foreign car that I could run into the ground.
On my current car & any car I own myself.
How often would I do oil changes? What would be things to check frequently for failure? How do fedex & ups service their vehicles?
Is the Lucus Oil Additive worth using or should I trust that the motor oil has the proper balance of additives?
 
Renting a vehicle would cost approximately $7200 per year.
Driving 140,000 per year do you think it will be cheaper to buy a car & have it fixed?
 
Any advice anyone has would be greatly appreciated! Thanks so much!
 
Chuck The Driving Man