The M03 (R,pro or whatever) was just a blingy version of the 03. It had some good hop ups, but nothing unique. The 03 has been out of production for quite a while, but spares are easily found. The M05 is the replacement. Opinions are heavily split on which one is better. They share gears, lower suspension, uprights, knuckles, axles and a few other bits, but are completely different cars otherwise.
You may want to check TCS rules, in case they've decided to disallow the 03.