I've never tried to figure it to the mile but my Pilot has always been easy on fuel. Though I run a modified setup (ported motor, torque pipe, dual intake, 39mm carb) I know I can go at least 30 miles before even thinking about running out. I'm not sure where the reserve fuel light kicks in but I almost always seem to be one of the last in a group to have it come on. Several of us have figured somewhere near a 40 mile range for a tank of fuel when riding under normal woods trail conditions.
You mentioned a no-rev limit cdi on your wish list. I'd be surprised if it didn't already have one.
And, before deciding on a set of replacement tires (even though that should be a ways down the road) I'd see how the current setup does, and ask around for opinions on what tread pattern and ply rating performs best for the terrain you'll be running in most of the time. You may find that a different tread pattern than what you have in mind initially might do better.
As for the who buys the gas for the Pilot
I'd say at your age you might think about trying to bear that load on your own! hehe Not to offend bu based on the purchase price (or lack thereof) I'd be hard pressed as a parent to buy the fuel for my (old enough to earn that kind of money) 16 year old. A major repair is one thing but you have to take on SOME responsibility if you are prepared to enter into the off-road world of Pilots. A new four-stroke bike or quad is one thing, but these 15 year old machines are quite another and maintaining and operating them is not a cheap endeavor.
Just some food for thought.