I'm going to try some logic here
If a defective speed sensor would return a speed of 0 mph then if that was a problem with the steering then the PS light wouldn't be on when you start the car. Also if the speed sensor is defective wouldn't it cause an error code itself? My Justy would trigger a speed sensor code if it was idling above 1500RPM for more than 30 seconds (normal in the winter when I was letting it warm up).
The clunk noises while turning is most likely coming from the C/V joints. (Take it easy around the corners or it will get worse fast)
Are all the PS hoses ok (no leaks)? When we changed the o-ring on my car and put everything back together my dad had me go through the full steering range a few times to make sure there was no air in the lines (another thing to check for).
I also assume your belts are tight and not slipping.
These are the few things I can come up with. I'm sure a few others out there will have their (better) suggerstions
