For 6 or 7 nights for 4 people I think your budget is pretty tight. Sure you will split the gas and lodging costs but it still only equates to $35 per day per person. On the right hand side of this page you will find the gas calculator that will work out how much each of the vehicles will cost you if you have an idea of mpg., but at 20mpg you would be looking at close to $400. As for the vehicle choice you will need to work out the pros and cons based on size for luggage, comfort, reliabilty and cost.

You can also get an idea of motel costs in areas you are thinking of stopping over by browsing the RTA deals that are available, also on the right hand side of this page. If you can get one with a breakfast included it will help set you up for the day, but at the moment I think you will barely have enough cash to eat, never mind going out for a beer or sight see etc.

Interstates are a priority to keep clear and open to traffic as are all major routes so even having to wait out a storm you will be fine. Keep up to date with weather and road conditions as you go and use the info wisely for the day ahead, there is no point heading up a mountain on a scenic By-way in a blizzard and putting yourself in harms way.