There are a handful of flyers in my indoor club with EDF models, and between us have over 40 EDF models, some are twins and there are a smattering of four motor ones too.
For my money you could do worse than buy a kit, EDF unit and depron parts from: http://micromodeldrome.com/semi-profile-for-brushed-edf30-69-c.asp
I know its more expensive than roll your own, but you are paying for speed (buy today - fly tomorrow) and success (I can vouch for the Hunter and Sea Hawk), and I know that depron may not be the way you want to go either, but you will learn most of what you need to know quickly.
I don't think that the champ motor will be up to the job, the P51 motor can usually cope, unless you get a poor one or it's already well used, better motors can be had here : http://micro-motor-warehouse.com/collections/all-motors
Oh and a duct can be a thrust killer in these small sizes, so good luck with that! You are spot on with your weight target, once you get to 50grams they can be a struggle to fly. Acceleration and climb rate will be low, and you can't do both at the same time, the walls come into play quite quickly and you need both speed and a little height to pull of the first turn, if you manage that it gets a little easier, but you will struggle a bit! You won't need three guesses as to how I know that.