I just did the oil on my 328, and it said 17k until service! My 2006 325 was set at 15k after a rest. I usually change it every 7k, and use the Euro spec Mobil 1 0w-40. Definitely do not wait the 15k+ factory suggestion. If you don't drive it that much in a year, do it once per year at least. You can get the filters for $10 or less on Amazon, and I've been buying the oil at walmart.
Get OEM spark plugs from one of the site vendors, it'll probably be about $15 per plug. It's also a pretty easy job.