There is nothing wrong with going Plus 1 on a E46 325xi (going from stock 16 inch to 17 inch). In this case I would recommend going with a wheel that has tightly spaced "spokes" so that you don't see the smaller brake rotors as much, i.e. I have Beyern Mesh 17s on my '03 325xi and it is harder to see that the rotors have a pretty good gap than if I had used a Beyern 5 spoke wheel in the same size.
Now, on my '03 330xi sport which has 17 inch wheels, the stock wheels will now see duty as the winter tire wheels and in spring, a new set of Beyern wheels will be fitted in the 17 inch size with Plus 0 summer tires. With most roads in poor condition, it makes no sense to go to 18 or even larger wheels since the tire sidewalls are so small and the tires hold so much air so that they don't do a good job at helping to absorb road shock(s). If you do want to change from 17 inch, only go Plus 1 (to 18 inch).