I am in the process of converting my RS4 as well. How did your 4300 kv motor work out? Does it have enough low end? I am keeping the 2 speed and the brake so I don't have to deal with the one way bearing for now. But I may eliminate the one way and the brake later to save weight. Do you have the file for your battery tray? I am going to do something similar. I like the Hobbywing EZRun Max 10 4000KV combo, but i feel like I am just guessing at what KV I should be using.... I do have several choices for pinion sets, but I am not sure if I am even close...

For the pinion I shaved off the stock bell and machined a interference fit piece of aluminum to press into the center to eliminate the bearings. Leaving enough meat out the back for 2 set screws to lock it in.