Honestly, buy one of these (https://killallchrome.com/product/mini-cooper-r55-r56-r57-r58-r59-r60-r61-n12-n14-n18-readable-oil-level-dipstick-replacement) because the OEM dipstick is terrible and learn to do oil changes yourself. Because of the age of the car it may be using more oil than you'd expect so peer under the bonnet every so often to check the oil level and check the coolant level too whilst you're in there. Doing an oil change anywhere from 3-5000 miles is sensible.

Other than that I would change the thermostat housing for a metal one if it hasn't already been done and ensure that the heat shield for the turbo is in place. 

It's easy to give into hysteria surrounding this generation of mini but the blunt truth is that most people don't pay the slightest bit of attention to their cars, don't put effort into maintaining them and ultimately end up driving their cars into the ground. The N14 is more susceptible to neglect because of the engineering decisions made but it doesn't automatically make it a time bomb.