triple chainring isis -> single square taper
i abandoned geared bicycles some years ago because of complications that i didn't feel like dealing with then (derailleurs, shifters, etc). now i'm rethinking things and want to convert a roadbike with 3 rings up front and 9 cogs out back to be a 1-9 setup. i know i'm going to be labeled a noob and get flamed or whatever you internet forum fools do, but i've done a bit of searching on here and haven't found the exact answers i'm looking for. so here goes:
truvativ isis triple cranks
truvativ (?) isis bottom bracket
shimano 105 derailleurs
9 speed shimano cassette
i wish to get rid of the front derailleur and 2 extra chainrings and replace the crankset/bottom bracket with a square tapered single speed crank/chainring setup. so...
what size square tapered bottom bracket should i use to replace the current isis bb? (i realize this depends on the size of the current isis bb, which i have yet to remove/measure, but i'm sure someone's done this, right?)
i plan on using a bmx crankset/chainring, so will there be any issues with dropping the chain? is there any way (the perfect chainline spacing) or a product (bash guard?) i can install to keep problems from arising?
i know people love to call others out when they ask stupid/naive questions, so if that's you, please, reply and i'll get a kick out of your hater speak. but...
if anyone on here has done this or has read a thread where this is answered, please help me!
thanks in advance.