Part 1 - What is the best car to convert ?

Next: Part 2 - What kind of motor should I use ?

There is no "best car" to convert. The vehicle of choice depends on what you want it to do. Porsche 914 conversions tend to be quick and have excellent range, but they aren't much good for hauling a couple of kids to school. Volkswagen Rabbits have room for passengers, but can lack pizzazz. (Not all though, check out Bill Dube's Wabbit).
Mini pickups can handle a lot of battery weight, but tend to be rather heavy. Basically, there are a few general rules:

1. Look for something lightweight with plenty of room inside.
2. Try to avoid anything over 10 years old, because parts availability starts to drop off. Just because a vehicle is free or inexpensive, doesn't make it an ideal conversion. If it was a junky gas car it will be a junky EV too.
3. Most importantly, pick a car you will want to drive.
4. Get a car that is rust free to start with. You can find the cream puff. Having to do restoration, i.e. paint or rust repair, will add significant cost to a conversion.

If you check out the EV Discussion List Photo Album, you will find over 1300 of the most varied conversions you can imagine.

There are basically two types of EV: using DC motor or AC motor. The following are basic structures of these two types of EV: