First of all, the BMW 316i is slow and under powered. It's fuel consumption is rather high given it's small engine. The better option is either the 318 or the 320.
As for maintenance, it will be definitely more expensive than a japanese car, especially when parts start breaking down as with any old car. Simple maintenance like sparkplugs and oil change is not the issue. It's the wear & tear of the car itself that you should be wary of.
You can also try reading the following thread about how expensive BMW maintenance can get.
http://tsikot.yehey.com/forums/showthread.php?t=72781
Recently we just replaced the ignition coil pack on my dad's 318i. That was P12,000 for parts alone.