It will be as reliable as the maintenance history on it.
The 330ci usually came with auto gearboxes, those are not reliable when not serviced (usually the case). The manual gearboxes are solid however. Engine is solid.
Cooling system also is not solid and would have needed at least a replacement on the thermostat, expansion tank and rubber pipes by now.
Actually, almost all rubber on these cars would have started to rot by now and needs to be replaced. There is a lot on the suspension system.
If you find a neat+well_maintained example that is not costing you an arm and a leg, its a great car.
Going price for these usually around R90-100k for the ci. This is the non-ripoff price. The ripoff prices are around 140-150k. A really neat example thats well maintained I would at most go to R110-115k, but there need to be invoices showing work done that makes that worth it though...
The 330i 's non-ripoff price is R75-80k.