There will be occassional lamp failures, there's simply no guarantee that you won't have one. However, bad lamps generally are an immediate failure or happen within the first minutes of use, this is why you should put some use on your gear so you can see that everything is functional. Considering the # of M500/900/910/96x & M3 Combatlights we've dealt with the # of failures have been quite small. I have no hesitation in recommending those models.
The mount situation is your call, the thumbscrew mount will certainly do the job and if you ever decide you need a throw lever mount it can be upgraded to the ARMS or LT mounts. In the case of the M900A w/ ARMS mount, it will be fine unless for some reason it's excessively too tight or too loose.
As far as where to buy, you can give us a try if you'd like.
Thanks,
Calvin
BrightFlashlights.com (website currently under construction)
Phone: (352) 732-2156 from 11am-7:30pm EST (Mon. - Fri. & until 4pm Sat.)