.5 : 1 power/weight ratio is a good starting point, i.e. if the model weighs 20 grams total, you'll want at least 10 grams of thrust. You can sometimes get away with less, but more is better (and more fun too!). The Parkzone gear will probably be your best bet, they're just so easy to drop in and setup, plus the customer service is pretty darn good if something goes wrong. There's pleanty of motor/gearbox/prop combos to choose from, but as you mentioned, its a good idea to get at least an estimated weight of the model.
Good luck with the project
Nick