and if you've ever used mechanical keyboards in the past, you can kinda know how they feel by the sounds they make and here is a vid on the sounds of each switch types
but what it really comes down to is actually typing on it yourself to see which you prefer...
the OCN favorite mech kb has got to be the ducky shine. but that's not to say others aren't good. some people swear by their CM quickfire and some people prefer the programability offered by corsair/logitech
you also should consider if you prefer the standard keyboard configuration or the 10keyless compact design. it depends on what types of games you play and if you feel you need the num pad at all.
as far as mice, here is a quick link to some good info on teaching yourself things you need to be aware of before making a selection
but AGAIN, a mouse can have the best tech in the world, it would still be a crappy mouse if it feels wrong in your hand or doesn't have the specific feature you are looking for such as programability etc..
monitor wise, I'd just suggest IPS with some decent response time.. i'm not much into 120hz since my eyes are too old to see the difference anyway. personally i'm sporting a 1600P dell and i'm happy with it. eventually i may look into a 4k monitor, but that's pretty far down the road. the bargain Korean panels are generally very good value. personally i wouldn't overclock them because again i'm not into the whole 120hz deal. but if that's your thing, then go for it. just make sure you get some good profiles to run if you are going to OC because the colors are going to be really off if you don't run custom profiles.