I don't know about the tech manual, but aftermarket manuals for BMWs seem way more abundant than for Benzes. I have a good Bentley for my 5-series (BMW dealer parts counters actually sell the Bentley manuals, but they're cheaper elsewhere).
The Bimmer forums I've found aren't nearly as interesting or informative as this one. For me, it's actually a bit of a bummer (get it?). Try starting at
www.bmwe34.net and check out the forums for the 3-series linked there. Also look at bimmernut.com. I haven't been there in awhile, but there might be something there. I'm restoring a 1974 2002 and I wish there were a support group on the web like this one for that car. Luckily, my local BMWCCA chapter is loaded with 2002 nuts with good ideas, tools, parts, time and beer.
For BMW stuff, look at Bavarian Autosport's website -
www.bavarianautosport.com.
Good luck.