The advantage of buying it at 15K is that you can acquire it for a good discount over anything new, and apply all the useful preventive maintenance work to ensure it's a good long-term proposition. The 235i/240i seems to be an overall relatively reliable model. The BMW Extended Warranty runs up to 100K miles - so that tells you something positive about how relatively reliable these cars are up to that point.
Take a look at threads like this on 2 Series
long-term maintenance for an idea. In the main, we're talking about annual oil changes, replacement intake charge pipe*, diff oil, gearbox oil, overflow hose and a few other things between 30K and 60K, to take you up to around 75,000 miles. This is stuff that won't be in the standard BMW service plan. You could also consider extended warranty. It's notable that extended warranty (in the UK anyway) excludes hose failures, so that implies recommendations to replace some of the coolant hoses around 50-60K are sound advice.
In my case, I'm the same as you about my 2016 235i 'vert, and am intending to take it to the 70K/10year mark at least. I don't see anything quite like it in the current BMW range or from anyone else and I like the car. I think that by the time it is swapped, it'll probably get replaced by an electric anyway.
[*more an M235i thing - apparently a better design in the M240i]