"...comfort is a big issue, reliability, mileage, and the ease and cost of maintenance and repairs. What's the best car?" You have too many competing variables here to give a definitive answer. One goes up, the other goes down. What's best for me may not be best for you. Will you be sticking to paved roads or off-roading up logging trails?
One criteria missing from your list is "safe". If you will be driving 20k - 40k miles that's what most Americans do in five years. You will be in five years worth of people not paying attention, drunk, text messaging, looking at their nav screens, screwing with their ipods, etc. Get a car with ABS, traction control and as many air bags as possible.
For $8k, I'd look for a 1997 - 1999 Mercedes Benz S500 or BMW 5 Series safe, comfortable and reliable.
No matter what you drive, putting 20,000 to 40,000 miles in one summer is a hell of a lot of driving. Do you plan on sleeping or eating?
PS: Spend a few extra bucks and get the Escort 8500 X50 radar/laser detector and ZR4 laser shifter before you hit the road.