I paid £500 for a 2.8 Z3 earlier this year - turned out I be a good car (after I tidied it up, put new plugs in to cure a lumpy idle and a new battery) I got very lucky a few months later and sold it with a good profit albeit that wasn't my intention.
Now here's the thing, after extensive discussions with the owner he stated the car was in great condition, was very recently in use, ran beautifully and didn't need much in the way of tlc. The lumpy idle could have easily been the head gasket, piston rings or a myriad of other things so I get very lucky and at £500 it was worth a punt as I'd get my cash back if I had to break it.
I'm assuming the Z4 you are looking at is considerably more than £500 and if the electronic steering is on its way out that could be an instant big bill especially if you aren't mechanically able yourself. There are lots of treatments that can hide engine, gearbox and diff issues just long enough for you to get the car and drive it a short while. There's also the fact that you will have to accept certain issues as it won't be cost effective to ship the car back - the seller will always argue that the car was "perfect" when it left them.
Unless buying from a garage, I'd never buy a car unseen not even from another forum member now matter how highly respected they are on here! No before I get pounced on I'll reason that most forum members selling a car to another member would want to meet face to face to go over and discuss what they believe to be issues with the car so the new owner is more than happy.
20 hrs is a long way away and the cost and pain of shipping the car back could mean you are stuck with it, a nightmare if it turns out that you really don't want it.......I'll leave that with you.