Hi, I was is the same situation as you about a year back – bought an Anthem X2 2010 frame to build. I put Reba RLT 100 forks on mine which i bought from one one cycles for £200, they weight 100g or so more than the equivilent fox 100’s but i really rate them on the Anthem – definately compliments the rear suspension for smoothness.
With the headset you need a reducer to accomadate 1 1/8 forks, which i was told only can be supplied by a Giant dealer (not sure if its true or not) it was a tenner and took a week to get in – i got my LBS to fit the headset / spacer and also the BB which again was a OE fit from Giant. Ive had no problems with the durability of the headset or BB hundereds of mucky miles later.
Ive got a Fox RP23 triad, awesome shock – i have it on setting 2 with no pro-pedal and thats good for just about anything in the UK – on the road pro-pedal is a good idea but off road it actually makes the rear a bit harsh. I wouldnt be too concered about the rear shock though as the meastro suspension is brilliant – i cant imagine there would be too much difference between Monach and Fox
Giant frame sizing is a stange thing and on my medium ive gone for a 80mm stem with a 6 degree rise and Easton monkeylight XC bars cut to 630mm, which is pretty much spot on for me handling wise
The Anthem gets used more than any of my other bikes to be honest and can kick the ass of bikes costing 3 times as much, you certainly wont regret building it up