An AC motor for a car might be in the 100-200 pound range or so. DC motors tend to be a little heavier. Along with the motor, you'd need a motor controller and cabling.
A general rule-of-thumb is that it takes about 100 watt-hours to move a thousand pounds of car one mile. I just started on a project to convert a Mazda RX-8 to electric. That's a 3,000 pound car (before the conversion), and I'm planning to use a Netgain Hyper9 AC motor, which is 120 pounds, which is pretty small compared to the rest of the car (or compared to the original rotary engine). Battery weight is the bigger concern; if I can keep the pack under 400 pounds I'm doing pretty good, and I'll be lucky if that gets me 100 miles of range.
According to Magnax no-load losses reduce the effective range of the vehicle by 10% to 20% (iron close to rotating permanent magnets equals braking when there is no load) Magnax claims 85% less iron losses due to the low amount of iron.
