It could be a few things so check first before buying.
The front hub axle could be bent.
The lower suspension arm may be bent or creased.
The rubber bushes may be shot in the suspension arm or other components connected to it.
The steering rack may have skipped a notch or damaged.
Steering rod damaged.
Broken bolts in lower suspension.
(worst case scenario is damaged chassis at lower suspension arm connection, but highly unlikely)
I had to check all of these when my dad took his car in to a kerb and then in to a 2 foot high concrete wall and in to the air eventually landing on it's side at about 45 mph. Made for some fun inspections but apart from the bodywork damage the only fault at steering was like what you have and it was the rack that took the knock and slight crease in lower suspension arm. Luckily I was able to just have to do some adjustment to the allignment and all has been fine for the last, close on 2 years and MOT's perfectly.
Replacing parts with secondhand ones is fine as long as they are in good condition but if you use anything with bushes in them then changing these is a good idea and will be easier while it's already off the car.
morgan105
