Batteries might not need to be replaced soon...it all depends on what age they are. They should have a stamp on them...letter then a number. The letter is for the month (ie A for January, B for February) and the number is the last digit of the year (ie 9 for 2009, 0 for 2010). They'll last 4-6 years depending on maintenance of them.
I'm guessing with the way you describe it, this guy is meticulous about his cart. That says a lot. That being said, I think BBB's are over priced and parts can be hard to find.
Any cart that is lifted with big tires, lights, back seat etc etc is worth at least $4500. 4wd and winch and other accs will only add $$. I'd say it is probably more in the $5-6k range.
Dawg2: "Go read the rules again. Slowly."
John Wooden: "Happiness begins where Selfishness ends."