Heh, why don't I place wagers on these things? I'd be rich by now :P
Bombardier have really got it right with the Aventra platform, they'll be absolutely omnipresent soon.
With regard to the bringing forward of the introduction, I imagine that's probably in part due to the unexpected overcrowding issues that arose when they introduced their new timetable objective. Extrapolate 6 years of organic growth on top of that, easy to see why they took the option to accelerate delivery as soon as they could. I'm curious what the extra units will be though - the fact they haven't already been seen to be doing this and have specified 2019 at the earliest makes me think they'll be something going off lease at that time, so possibly ex-GA units?
As for 100mph, pretty sure that covers every unit - 357s are 100, 387s are 110 and the other units they are presumably looking at would also be 100. I've often wondered if there was any scope to up the speed between Upney and Benfleet to at least 90, if not 100, but I don't know the route east of West Horndon well enough to know of any reasons why not. I can't ever see the branches being upgraded, I don't imagine the calling pattern warrants it.