I think if the car is in fairly good condition to begin with, and assuming maintenance has been done prior, 1000 to 1500 is fair in terms of annual maintenance cost.
If you do your oil changes as often as you should, likely 400 bucks is around where I spend on oil changes every year alone. I change my plugs, air filter, cabin filter, once a year as well, and this about 100 to 200 depending on what brand/quality parts you are buying. There will be other common wear items here and there (shocks, breaks, tires) may not need replacing every year, but do have weight.
You can save money by doing most of this work yourself. You can also save some by cheaping out on parts, but I wouldnt recommend it. You are considering a premium car, make sure you put premium parts in it.
In all honesty, this is stuff you should be doing for any car, only the part prices for the e46 will be a bit higher.