Don't get an N55. Performance gains and many more years of R&D on the N54 allow it to make substantially more power than the N55 counterparts at this point.
I once also was a big BAIC fan and wanted to get into an STI. But things happened and I ended up in a 135i.
In terms of the Bay Area 1addict scene, its fairly quiet and not much goes on around these forums. LUCKILY though, the sister site e90post has a very active California section.
Even better yet, 1-series and 3-series are very similar mechanically. Things like downpipes, intercoolers, chargepipe & bov's, even suspension like coilovers & strut/spring combo, down to the bulbs in our angel eyes are exactly alike. So once you pick up your 1-series, you are free to check the forum marketplace on the much more active e90post. (once again, they are the same site! you even use the same login, and share the same PM box).
Also, in regards to your DCT comment. I'd recommend you stay with the tried and true 6-speed MT. Rowing through gears is unrivaled to pressing paddles on the steering wheel. Especially since you came from the STI crowd which only came in a 6-speed Manual. Not to mention later on, if you do get bit by the mod bug. You wont have to worry about all the transmission issues that could come up. All you would need to do is get an upgraded clutch. A good example of this are with all the single turbo & even RB (upgraded twin-turbos) 6-speed AUTO having issues with their transmission giving out on a 140MPH 5th gear pass. Check it here : http://www.e90post.com/forums/showthread.php?t=769959
Lastly in regards to shops for engine work. Our N54 platform is still fairly new and as of now, the only power mods available are as listed: Tune, Intake, Downpipes, Intercooler, CP & BOV, Meth, E85 and lastly upgraded Single or Twin turbos. As you can see, none of these mods involve much more than simply unbolting and installing new parts in. There are no aftermarket upgrades in the block itself yet. So any shop around the bay should be able to take care of you. If you are really keen on getting work done at some more well known places, I'd recommend Sonic MS in Mountain View.