A few thoughts to add onto Buck's excellent advice.

First, sleeping in a car is NOT an option with 4 people. Two of the biggest enemies of a trip like this are lack of rest and lack of space. There simply is no way for 4 people to get any kind of rest while sitting in a car. Camping is an excellent idea to save money - but even that does require money. $20-25 a night is pretty common for an average campsite at a state park or similar place. Even there, I'd recommend you budget at least a night or two for every week of your trip in a motel, in case you get bad weather, or just want a real bed for a night.

Speaking of camping, a roadtrip is not the place to see if you like camping. If you don't have experience camping, going on a weekend trip together before a long trip on the road is highly recommended. For that matter, a weekend warmup trip is highly recommended in general, so you can get a feel for what you like, don't like, need, don't need, etc.

Your transportation is something you need to figure out. There is a big difference between a 2 year old SUV and a 30 year old sedan - and I'd have serious doubts about taking a 198x anything out on a trip like this. Even if it is well taken care of, there are things that simply wear out from age.

For food, it is easy to underestimate how much food costs, especially if you don't often buy groceries yourself. $15 per person per day is close to a minimum, if you are cooking nearly all your own food.

Finally, I'd can't recommend enough that you take the compatibility test that Buck linked to. Yes, I know you are all the best of friends in the world, and nothing would ever change that - except that life on the road is different, because you are all packed together with very little time apart. Plan for conflict, and how you will deal with it. Roadtrips can and at times do damage or even destroy friendships, and it's the people who say "it will never happen to us" are those most likely to have a trip that ends poorly.