I would be a little cautious with the diagnosis. These racks are not well known for leaking. But other things are very good at leaking onto them giving that impression. 60k miles is absolutely nothing for a rack either. These things can go on for 150k+ miles with no issue.
You may well find the issue is dealt with when the car comes back. But that might be from 'incidental' replacing of another fitting which was the true source.
I've just dealt with a leak in that area. It was actually a distorted o-ring on the fluid reservoir from over-tightening and it had crept out from the cap. That's certainly one reason the reservoir can look so messy although there is a breather hole in the top anyway.
I have a couple of spare racks. In fact I nearly sold one recently to someone who was told their rack was leaking. I gave the same advice as above and he came back and said I was right and he no longer needed it. Which was fine with me of course.
Recon racks are available on ebay for little more than I would want for a secondhand rack. £160 v £100. Of course I don't know how good the service is but these guys' feedback is exceptional. Secondhand racks are valuable to the 3 Series modding community as they are 'quicker' (less turns end to end) than the factory rack so in high demand and hence price.
https://www.ebay.co.uk/itm/BMW-Z3-1...k-Repair-Remanufacturing-Service/272737811956
If you replace the rack, even with a second-hand one, I would also replace the inner and out tie-rods. These aren't expensive and the tracking will have to be reset anyway. And it probably goes without saying using fresh fluid.
If I'm honest I'm not sure it's the easiest of jobs if you aren't used to using spanners.
