It's not your standard running costs, it's when the suspension needs doing, it's if you want to get the VANOS issue fixed.
As a rough guide Fox has a similiar aged car to the one you would be looking at, with the same engine (although his a 5 series) and his is one of the best conditioned cars you'll ever see, yet it can regularly cost him 4 figure sums every few months to get everything sorted.
It's unlikely to break down and leave you stranded but the actual cost to keep on one the road in a decent condition with everything working is pretty big.