Hi Rafaela:
I would very much recommend the E-Flite 15 motor paired with a 2,500 mAh 4S Hyperion battery (Hyperion batteries are available from allerc.com). I'd also recommend a Castle Creations Edge Lite 50 ESC and a Hubin FM-9 timer system (You'll also need the programming box from Will Hubin).
I have been using this exact system in my Gene Schaffer-designed Stunt Machine I (also known as the "Oosa-Amma" - long story...), and have found it to be flawless over a long period of time and a lot of flights. You might be able to get away with a 35 Amp ESC in the Ares, as it is a smaller plane than the Stunt Machine I. Castle Creations has a 35 Amp ESC that they call the Talon 35.
I'd pair this with an APC 10 x 5.8 EP prop. You might also try an APC 11 x 5.5 EP prop, but the Ares has very little ground clearance even with the 10-inch prop. We've tried both props on smaller ships like the Ares, and have found the 10-inch prop to be the best choice.
Hope this helps - Bob Hunt