The electric steering can be troublesome on the E85 but there is a fix for when the steering becomes stiff to turn.
@Lee did an article posted on Z4 Forum which tells how to adjust the steering column.
This is another showing the same fix on YouTube.
View: https://www.youtube.com/watch?v=gWIuYpXCLjI
There is nothing to service, as such on the steering but this is where your mechanic should start. No parts cost involved and really no more than 1 hour labor.
Tony.
ps. A new battery needs to registered to the car after 2002, which could also put out those warning lights on the dash.