Any guess would be a good guess but I'm going to say I wouldn't keep it past 60k. That gives you enough time to get some of your money back before the residual value drops off a cliff because of reliability issues around 80k.

Just keep driving it if you have no problems. Depends on your driving, problems can start to appear around 100k miles. 

Generally you should be alright for a while. The BMW electronics issues don't tend to show up until you're pretty good into 6 digits. The N54 is inherently expensive to maintain given how technologically advanced they are for their time. Turbos, high pressure fuel pumps, and injectors are all extremely common failures that are quite expensive to fix on these cars, those usually start to go out around 70-80k miles minimum. But generally, you shouldn't have to start dumping too much money into it before then. Seeing that you only drove 30,000 miles in 11 years, you should get plenty of use out of that car before you would really *need* to sell it. If you keep it for a good while, you'll start to experience some typical BMW oil leaks, but if you don't drive much and maintain your car well they likely won't be bad, maybe they won't even happen. I say have fun with it for a while, they are really fun, fast cars and you've got yourself quite a low mileage gem there. Getting rare to see an old 1 series with less than 100,000 miles nowadays.