I've bought my Z4 from eBay in April 2016. Paid around 5k. Since then replaced the whole suspension (I mean everything, even a bolts and nuts), brakes (I mean everything, from callipers to disks, pads and hoses) , all 4 wheel hubs, various belts and rollers, countless little thingies like alloys refurbishment or new radio. I think I've already spent on the car something around purchase price. But now it is mechanically like new. Still need to do some exterior and interior stuff. When I bought it it had 84k miles, now 102k. Never ever before I've spent that ridiculous amount of money just in first year of ownership.
But, I never regretted this purchase. I went for e89 test drive when they was available on dealerships, but found that e85 is much more my car.
So, be prepared to spend heavily, or if not - better look to tidy e89 on finance. Could turn to be cheaper. These days it is hard to find perfect e85, they are either abused and neglected or not on sale.