You're probably right that this far out the prices will be crazy. But, its not too early to start looking. What I would do is find something and reserve it. However, under no circumstances should you do anything binding (pre-paid). When you reserve, all the agency will want is your name and email...there's no credit card hold or anything like that.
The other thing you can do is start gathering discount codes. If you're a member of Costco/BJ's,
AAA, Entertainment, etc., there are usually discount codes to be had. You should plug in rental choices both with and without the codes. Sometimes no codes works better.
Of course, this far out, you probably won't see too much to get excited about. The other gotcha is that rental codes, with the exception of Entertainment, tend to run out at the end of December. Since you're looking at January, you may have trouble finding codes that will work.
The key to car rentals is to constantly keep checking. Keep something booked, but don't stop there. As you get closer to the date, you'll find that rates start going crazy. Sometimes they'll jump way down, sometimes up. I sometimes go in March and find that I get the best rate somewhere around 2-3 weeks out. I've even seen where the rates drop a day or two before, but never count on that.
The good news for you is that January (maybe with the exception of Jan 1 and MLK weekends) should be pretty quiet and I think you'll find some good rates on a minivan. At the very least, I would think you'll find a deal < $300 and you may do better than that...
