I would go with 3mm. All my rack mount enclosures always have a 1/8" thick front panel. As far as the blind holes go, go down 2.25mm deep, that is plenty for a standoff. What I prefer doing is instead of using a stand off with a male thread on one end, I use one with two female holes and then just use a set screw in the hole in the panel, that way you engage more threads since you dont need a thread relief on the stand off. I use this method on 2.5mm thick panels and it is solid as a rock. I have also had much better success with the Loctite version of JB weld (Just called Loctite Weld). After battling with JB weld with my MB6582, I dont use it for stand offs at all. A mechanical fastener will always be much better and secure