Would a 3 kw electric car motor suffice, seeing that I am content with a speed of 30 kmph (about 18 mph), if my car weighs 650 kilos and with one passenger would weigh about 725 kilos?
How much would the right motor cost and where can I get it from? How do we mate the motor to the wheel?
The KW depend on the voltage of your system, but if you're talking about running your electric car at 72v or so, 3kw should do just fine. When you describe your light conversion and your speed requirements, a golf cart motor comes to mind, like the NEVs use.
I'd get a golf cart motor on Ebay, personally. The price depends on whether you buy it new or used, how far you're shipping it, and such. I saw one today for 550 USD that would suit your needs.
On mating the motor to the wheels: if you're intending to keep your transmission, you'll build an adapter plate (or have it built). There are some good conversion manuals out there to explain the details.