Agree with above idea! when riders turn up to get a coaching session just ask them what size their bike is and do they want you to do a free performance evaluation check ❓
3x tail whips 5x drops 6x jumps and assorted manuals backflips and other shit later, hey presto! their bike is checked and you know what to buy!
I like the new style Giant Trance frames, but I love the Trek Fuel EX DRCV frames.
I have mines set up with a 24 38 rings, xt brakes and crank and some reasonably tough components, including some mavic ex325 rimmed wheels and hope bulb (my favourite) hubs.