I have an MPS-1A motor system from DJ Aerotech in a Guillows' "Arrow" rubber band contest free flight kit, with a 2 cell 250 mah Li-Poly pack. Just added a small rudder and elevator on the tail section, with some small carbon rod pushrods (because I just happened to have carbon rod---small music wire would work great. too). A pair of Pico servos and GWS R4P receiver, Pixie 7P ESC. Flies around at less than 1/2 throttle cute as can be!! Can be trimmed to fly in a beautiful slow climbing turn hands-off. 45 minutes flying time!!!
Their MPS-2A (twin motors driving a common prop shaft, wired in parallel) would be a great choice for such a conversion for models a bit heavier, such as the Guillows scale kits in the size you mention. Combined with a 3 cell Li-poly 250 pack, it would definetely do the job at much less cost than any brushless system.
If at all possible, go with Li-Poly power. The weight savings on small models like this has a HUGE effect on performance. Weight is extremely important on this size plane.