ABS Im sure will be a complicated installation and be expensive. I think you really don't need it, just stay calm while drivng and pump in brake a few times if you need to stop in a hurry to prevent the wheels getting locked.
In an emergency brake (without abs) the wheels always get locked. You can do whatever you want.
The advantage of having abs is that you can push the brake pedal as hard as you want and you are still able to turn the car around an obstacle without taking your foot off the pedal (the wheels don't get locked). This is really a big advantage.
So is abs needed? Yes it is! (no newly sold car without abs in Belgium, it is standard on all cars)
Can abs be added on a car without abs? I really don't know