Nice car mate, well done. Not too pimp ... I love the wheels.
Do you know if the steering issue only arrises when you are turning ? Stupid question I know, like why would you steer if you didn't want to turn ! But I'm wondering if you've got a slipping v-belt on the power steering pump pully. What kind of condition are the belts in ?
Another thought I'm having is electrical. X5's are very electrically complex and someone has changed the steering wheel. Very easy to knock something out in such an exercise. Does your airbag work by the way ? Another stupid question ..... I wouldn't recommend driving your car against a wall to find out !
Check the power steering fluid. Is it topped up ? Possibly there's air in the system and it might just need bleeding.
Good luck with it. Let us know how you get on.