jak1366 - I just purchased a 550i xDrive with similar options yesterday... so I can't speak to reliability. However, having owned BMWs for about 13 years, I can say I wouldn't be overly concerned about the specific build date unless you've identified an option that you know to be available (or not) in a specific production run.
My understanding is that the M Sport Suspension is only available if you specifically choose the M Sport package. I believe you just get the standard (albeit improved) suspension, otherwise. I opted for the Dynamic Handling Package (DHP), although I got the M Sport line - it removes the sport suspension and replaces it with the active adaptive dampers. This may be subjective to some degree, and both suspensions are likely great, but I opted for the DHP for two reasons: 1, it has the ability to "tone down" the suspension if I need to drive on rough roads (which is jarring with my existing 335i sport suspension), and 2, it includes active anti-roll technology that helps keep the car flat in a corner.
As far as reliability goes, as I said I can't speak to that - just picking mine up next week. However, I've had good experiences with BMW in the past 13 years... although there's an odd duck car even in the best and most reliable models. With the 550i I expect the issues to be more frequent and more expensive than the 3ers I've owned in the past; however, BMW has a FANTASTIC warranty and service plan for the first 50k/4yrs.
If you use Geico, I'd also look into their Mechanical Breakdown Insurance (MBI) if you plan to own the car outside of warranty. It needs to be added before 15k/1yr I believe from new. It costs a few bucks a year (nothing substantial) but protects the car from any mechanical or electrical issues for 100k miles or 7 years. There's a $250 deductible if you do need to use it, but that's a small price next to a $4k or $5k repair bill if ever faced with one. Thankfully I've never needed to use it, but the reviews from people who HAVE needed it are fantastic. No, I don't work for Geico...
Anyway, best of luck with your purchase!