Having recently gone through pretty much the same scenario as you, here's my lowdown on what I've found out in the last year..
10-speed is dominating, 9-sp (which I use) is still the standard for a lot of bikes, but getting hold of 3x9 shifters (3-ring front rings, 9-sp cassette) is getting much harder. Most people seem to be shifting to a 1-2 speed chainrng these days..
Don't buy a '10sp front mech' if running a 9sp rear, a 10sp front mech still only has three shift positions, but is designed for a 10sp chain, which is narrower gauge, and therefore rubs.
5-bolt chainrings are hard to find(!) ever since Shimano switched to 4-bolt..
SRAM top-end stuff (X-9/X-0) is lighter and better value than Shimano, but a little more delicate, so it depends on what terrain you're going to be riding!
SRAM shifters use a different pull ratio (1:1) compared to Shimano, same as on the road, so hence why they're not compatible..
Also like the road kit, you have long/medium/short cage. Generally speaking these are for 3/2/1 speed chainrings respectively, but there is some overlap.
Shimano chains still break with very little effort. SRAM chains rock, and use a quick-connect system, which means no more need for chain tools!
Wheels can be specced these days with a disc hub and a v-rim, which means a) you can switch them over between disc & non-disc bikes and b) you can keep your wheels if you change frames later, and easier to sell if you want to change the wheels. Yes, you will be carrying some extra weight, but it may be worthwhile. There's little to no price difference between a disc and non-disc hub, as discs are more prevalent, the costs come down!
Forks: Never used a Reba, but I have used Magura, they do a range for light to heavy use (I have a set of Odurs and a set of Menjas), and are VERY light, but tough forks, 80mm travel available and with the added bonus that their stanchion lengths for the short-travel forks are also short, so it doesn't mess with your geometry!
Most of all, many of them come with remote lockout - lockout is essential in my book..
Air forks - they work nowadays!
Scandium - it's the new titanium!
And one bad point about modern MTB's.. Bottom Brackets - you'll need a new thread for this.....
2013 On One Scandal | 2011 Ghost ASX 3700 | 2008 Magura Test Bike | 2006 Raleigh RS950
1999 Voodoo Hoodoo | 1999 Raleigh RSP 450 | 1997 Giant ATX-D 990 | 1997 Ghost X-700
1995 Giant Cadex MCM-3 | 1995 Voss Ti | 1993 Giant Cadex CFM-2 | 1992 RS Topline Mammoth