I think you will spend his college tuition before he ever drives that pathfinder. considering cost. even if you do it your self. new engine $1000. New trans $1000. on a trail you'll need at least newer used (less milage gears) $200-500. While your rebuilding your engine you should atleast get new plugs wires water pump ect. think about it. 600 now seems ok. But you'll easily spend $3000 getting that trail ready.
save up for either a different nissan or get a jeep. you can get a 92 or better with less than 130,000 miles on it for around $3000. it'll be a 4 cyl. but who cares. wish ya luck kid. Also just wait 3 yr old. Your gona spend all your money on clothes and food. thats when they start to grow.