The biggest issue with electric vehicles is weight. Accelerating and hill climbing both take energy in proportion to weight. Batteries are really heavy; you end up with a lot of capacity just to haul around your capacity. Batteries are also expensive, tricky to charge correctly, and have a limited lifetime.
This is why the electric and hybrid production cars go to such great lengths to reduce weight (both in the batteries and in the chassis) and to use really expensive batteries.
You can cheat the game by starting with a lightweight vehicle. The less your chassis weighs, the less battery you need to power it, so the less battery you need to haul your batteries.
Your real goal isn't to get a car to your destination, it's to get yourself there (most of the time; sometimes you have passengers or cargo.) Your body weight is a fraction of the vehicle's weight!
About the best you can do here is an electric bicycle. The base weight is minimal compared to your body weight, so now all your energy is going to your main goal: getting you there.
Instead of converting a whole car to electric, consider converting a bicycle and using it for your shorter trips. You may be able to get 80% of the benefit for 20% of the cost.