I would say the transponder first since it's hard to race without one. I wouldn't worry a whole lot about the radio. As long as you know how to access key functions you're set. It's not one of those things that'll necessarily make a beginner faster on the track. No experience with the losi rtr ESCs.. is it having issues? Starting out focus on your driving and just make sure you have the right tires.. whatever everyone else is running. I'm being hypocritical but you could save a lot of money and have just as much fun without marginal upgrades.