We've put a lot of money into the car over the last few years, with a lot going into the suspension (our roads suck) but also tires, brakes, new thermostat, the list goes on. It's in good shape at this point, but we'd like something that gets better gas mileage.
Putting money into a car as it ages is just part of the car ownership process, and purchasing a Prius with 4,000 less miles on it than your Fusion won't let you escape that fate. Knowing that any vehicle is going to break down over time, I set aside ~$50 a month into a preventative car maintenance fund to pay for any vehicles repairs, maintenance, car insurance and car tabs. I've been doing this for a few years and it really reduces the stress when things begin to fail on the car.
What's your primary motivating factor for getting a new vehicle? Better MPG to save money? Eco and environmental reasons? For traveling to the vacation spot? If your main goal is to save money, there are a lot of alternatives that don't involve purchasing a different vehicle — namely, learning how to perform your own car maintenance.
Also what sort of cost comparisons have you done? Are the insurance rates lower or higher for the Prius or the Volt? How much are registration and yearly car tabs going to cost? Will the Prius need service (tires, brakes, struts, etc.) in a few months? Speaking of which, how is the Prius going to handle the crappy roads near your house?
It also seems like there's a bit of uncertainty in what you want out of a new vehicle. If that's the case, it'd recommend thinking it over a bit, taking a lot of test drives, scouring the internet to learn more about the vehicles and renting a car for a week to ensure it meets your needs. Who knows, you may take the Prius on a test drive for a week and you might hate it or the novelty might quickly fade.
Lastly, check out
https://www.carcomplaints.com to get a better understanding of some of the issues you'll likely see with a new vehicle.