The good thing about car rental reservations is that they're non binding. You can cancel at any time or even not show up without getting charged. So if you want to try to do your budgeting now, go out and find the best deal you can and book it. At least then you'll know that is the most you would have to pay.
But, since travel related things can change frequently you should keep checking and looking for deals periodically. If you find something better than what you have booked, then book that and cancel your first booking. If you keep doing that, by the time you take your trip you should have the cheapest possible option.
The only other thing I'll note is that some companies have started giving you a discount for prepaid rentals. Some of the discounts can be pretty good, but you have to pay for those at the time of booking and you can't cancel or change it later without paying a fee. I would steer clear of that for now and just book a regular cancelable reservation to start with. Then, when you're a few months out from your trip you'll have a pretty good idea of what the rate for a car will be and if you see a really good deal for a prepaid rental, then go ahead and book it.
The other thing you can try if you don't find a good prepaid deal within a week or two of your trip is priceline. Again, by then you'll know what your best deal is, so make sure you bid somewhat lower than that, but you might be able to do even better there. But again, no need to try priceline until you get a lot closer to your trip.
HTH,
Brett
