I have found the world of brushless motors a bit confusing. I now own 4 of them. There is no standard, as far as I can tell, between different manufactures products. The best thing to do is find the weight of your model and go from there.

I think multiplex planes are on the heavy-side for foam planes. It may be cheaper to replace the plane at the same time, depending on your experience. You are probably into $200 for a moderately sized motor and ESC. Add the cost of Lipos and a charger and you are getting pretty pricey.

What type of flying do you do? What is your skill level...i.e. beginner, intermediate....etc..?

Once you go brushless, you will stay brushless. The same goes for Lipos.

Good Luck!!