Hi Noobie.
There is an excellent thread on the cost of maintaining e36 in this forum that you should check out.
My opinion, the cost of maintenance would vary depending on the condition of the car.
A well-maintained, good conditioned car would not cost much in maintenance. What becomes a money-pit are those cars which have been thrashed and/or neglected.
Every component in any car has a known lifespan (aircond, gearbox, etc, - search the forum for more details). Get these sorted out when the lifespan is reached, or it will lead to failure in other parts, overall costing more when it is eventually sorted out. These become the "constant repair" cars you heard about.
Your best bet, get a car from a fellow enthusiast who have been keeping it in good shape, paying a bit more if you have to (this saves you money in the long run, so its works out cheaper). Getting a car from a 2nd hand car dealer is the riskiest approach, as you do not know the owner's habits and maintenance record.
Many cars end up for sale because a major repair/maintenance is due. Be aware of this. That's why maintenance records are invaluable.
Good luck.