I have now completed a number of RTR conversions of steam outline models from OO to EM and would echo Alan’s comment about starting off with an inside cylinder model first, simply because it is an easier introduction to the world of conversion, with one less complication to worry about. When I first started converting locos, I followed the Pete Hill blow by blow accounts, some of which have been on EMGS websites but most of which are accessible from the Alan Gibson website. They are very clear and well illustrated; once you have mastered the first couple of conversions it becomes much easier to spot potential issues on future ones.
In terms of outside cylinder locos with outside valve gear, be aware that tolerances around the leading driving axle are very tight (due in large part to the wheels being pushed out to an extent not envisaged by the manufacturer). That means to have any real chance of success, the leading wheels need to have sufficient washers so lateral slop is almost eliminated (but still allowing the wheels to turn freely) and frequently the crank pin arrangement will need filing down to avoid hitting the back of the valve gear (assuming you want to retain the proprietary valve gear). Again, Pete Hill covers this very well in his conversions of outside valve gear locos.
I hope this is of some help.