If you are running an M03 (still very quick in the right hands) then I would keep it cheap as chips. If running brushed then any 23t motor/esc will do fine (Ansmann clash, Modelsport cheap brushed esc).
http://www.modelsport.co.uk/modelspo...products/31775
If you are brushless then don't go for any faster than a 13T - the etronix/SkyRC Leopard (both identical) combos are excellent value. I've been using 2 of them over the last year and can't fault them:
http://www.ebay.co.uk/itm/Leopard-TO...item3ccadc2352
If you want to run LIPOs then an M03 will need the round cased variety, something like this:
http://www.modelsport.co.uk/voltz-25...roducts/367906