I suggest you avoid purchasing a hybrid unless you have alot of stop and go traffic. Even then, I think you are long-term better off buying a non-hybrid if you are talking the Prius size. Hybrids don't make sense if you drive alot of highway.
Do the math. Compare hybrid vehicle cost to non-hybrid in similar size vehicle. Decide how long you intend to own it. Find out from a dealer or mechanic when the batteries normally go out and how much that repair bill costs. Add that to your vehicle cost. For that reason, I would not buy a used one since battery life is already counted against the value (unless you prorate a knock down in price accordingly). Concerning future battery replacement costs -- you'll either pay it someday or it will come off the value when you sell it. Figure out your yearly mileage driven. Compare gallons saved using a hybrid versus regular gas rig. Then do the math and see if it really is worth it.
I think in the coming years we will see the design foolishness of building cars that undergo many engine stop/start cycles in any given trip, unless such applications involve engine pre-oilers (which I don't think any do from the factory). I've read stats that say 50% of all engine wear comes from starting it, until engine oil pressure comes up. So hybrid owners might not only see high battery replacement costs, but also engines that don't last as long as non-hybrid vehicles. I don't think the engine controls on a hybrid allow easy interface with pre-oilers, since these oilers involve tying in a solenoid to the ignition switch.
For those readers who have an almost religious affection to things that are "green", I posit that "MONEYis a resource too". And for everyone (except congress), this resource is in short supply! (Sorry, I had to add that!)